You could save the cost by very simple circuit only for the purpose to gain the same power.
However, you need to be careful in selection of good power supplies as there would be critical points as shown below.

<< Output voltage stability against input voltage fluctuation: >>
AC 100V is suppled to general household, and sometimes this voltage may fall. Good power supply can keep the output voltage stable when the input voltage falls.

<< High efficiency: >>
20 to 30% of total input power becomes waste as heat and is emitted in PC power supply resulting in short life due to temperature rise of peripheral devices (Figure 1.14).

The higher efficiency is, the better power supply is.
But complicated circuit such as active filter to suppress harmonic current is required to reduce the efficiency.

Figure 1.14　Poor efficiency generates lots of heat

<< High power factor: >>
Power factor is described in the ratio of superficial power (apparent power) to actually consumed power (active power).

When power factor is low, input current of power supply increases to result in poor utilization of power distribution installation. Input current waveform becomes discontinuous as input voltage changes because the current flows into electrolytic capacitors (waveform of low power factor).
Effective countermeasure is to adopt active filter (called PFC: Power Factor Correction) to gain high power factor.
Power supply with active filter installed is expensive, but it plays an important role in power factor improvement and in meeting harmonic current regulation (IEC61000-3-2) as well. It also serves as stabilizer at low input voltage. (Figure 1.15)

Figure 1.15 Current waveform comparison between low power factor and high power factor
Current waveform of low power supply is discontinuous and contains a lot of harmonic components. Harmonic components are called electric pollution to give bad influence to power distribution installation.

Presence or absence of voltage selection switch indicates the presence or absence of active filter (Photo 1.1). When voltage selection switch for 100V or 200V system is mounted, No active filter is mounted. When input voltage for 100V and 200V system is available without selection switch, active filter is likely mounted. For relatively small power (70 to 150W), passive filter corresponding to harmonic current regulation may be installed. But it brings heat generation and weight problem.

Photo 1.1 115V/230V selection switch

<< No noise generation: >>
Noise patterns listed are "audible noise (acoustic noise," "noise which leaks externally along with AC line (feedback noise)," and "emitted noise as radio waves (radiant noise)." If those are at low level, the power supply is good. As countermeasures against noise, AC line filter can be listed. Chassis thickness or assembly method of power supply also contributes for improvement of shielding performance.

<< Small inrush current: >> Inrush current is the current which instantaneously flows into power supply shortly after input voltage (AC) is applied (Figure 1.16). The smaller the level is, the better power supply is. (You might have an experience that the light in the room went dark shortly after you turned on the light. This is caused by inrush current, and it may be close to 100A). To prevent this phenomenon, inrush current prevention circuit is installed in general. As the examples for that, power thermistor (Figure 1. 16-1) is used, or limiting resistor is installed in parallel to thyristor that is shortened after particular period (Figure 1.16-2)

Figure 1.16 Inrush current

Figure1. 16-1　Power thermistor method

Figure1. 16-2　Thyristor method

<< Small leakage current: >> Leakage current is the current that leaks to the earth from power supply (Figure 1.17). The smaller the leakage current is, the better the power supply is. Large leakage current causes safety-related problem such as electric shock. (Balancing between noise suppression and leakage current is a challenge as smaller leakage current increases feedback noise and radiant noise. In particular, medical standard requires 0.5mA max. at AC 100V.

Figure 1.17　Leakage current

Also, at single-phase two-wire system (Class II appliances) in Japan, leakage current regulation requires 1mA max. as its guideline. Moreover, low level is required to prevent malfunction of leakage current breaker. In general, our power supply restricts to 0.5mA max. at AC 100V. However, caution is that some power supplies from abroad allow 3 to 4mA.

<< Small ripple voltage: >>
The smaller fluctuation of DC voltage supplied to PC is, the better power supply is. Ripple voltage is fluctuation component synchronized with frequency of input voltage or switching. It is overlapped with DC component of output voltage (Figure 1.18). Spike voltage is specified as well as ripple voltage. This is caused by ON/OFF operation of switching devices such as FET, Diodes, etc. in hair-shape and also called spike noise.

Figure 1.18　Ripple

<< Small fluctuation against temperature change >>
The smaller fluctuation against operating temperature is, the better power supply is. Characteristics of individual components changes as temperature changes causing output voltage to fluctuate. Power supply needs to stably start up even at low temperature as power supply is likely hard to start up at low temperature. Some power supplies do not start up in the cold winter.

Our products have been confirmed to be able to start up even at -10 degrees C to -20 degrees C by extreme testing although the specification says 0 degrees C in general.

<< Long life: >>
Power supply is a relentless component that contains fan motor as moving parts, and electrolytic capacitors with limited lifetime, also generates heat. Lifetime of power supply is shortened by slower speed of fan motor and capacitance drop of electrolytic capacitors. To know the lifetime of power supply, reference data is fan lifetime(*1) and operating temperature of electrolytic capacitors(*2).

*Note 1: Description in specification such as 50,000 hours of life expectancy. The longer, the better. However, description in specification is the result coming from comfortable environment.

*Note 2: Description such as 85 degrees C or 105 degrees C. Our products use 105 degrees C or higher of long life capacitors at least. The higher, the better. The lifetime shortens as the temperature of electrolytic capacitors rise. Cooling system to reduce the temperature rise in power supply design is required.

Reference: Critical cause to shorten lifetime is "adverse effect of dust." Dust entangles fan to lower speed, and also rises the temperature inside power supply not to work until natural lifetime. In installation of PC, place the PC on the table several centimeters high from the floor, rather than placing it directly on the floor. Because if the PC is placed on the floor directly, it inhales more dust. Placing on the table improves dust problem much.

<< Others: >>
Is protection for overcurrent/overvoltage properly installed?
Is compatibility with the motherboard good?
Is countermeasures to prevent external noise or lightning surge implemented? Is protection against vibration or mechanical strength during transportation enough?, etc.

Meeting all requirements above needs lots of components, selection of high quality parts, and also complicated structure. It does not make much difference whether the power supply is cheap or expensive when it is used as PC power supply.
Expensive power supply does not even urge the PC to work speedy. That said, it is not until reliability of the product is cleared up and derating is secured to meet this with component selection that you can build the computer system to use with ease.
You need to understand that power supply is decisively critical component that decides the reliability of your system.