The main differences between a cheap psu, and a more expensive psu is quality, and reliability. Whilst the voltages my be the same, the ability of a cheap psu to supply that voltage/amps under load is not.
A good quality psu will normally achieve an efficiency of around 70%, but a cheap one will struggle to reach anywhere near that.
A good quality psu, is one of the most overlooked pc parts. You only have to look at these forums, to realise a lot of pc problems are caused by cheap unstable psu`s.
By getting a cheap psu, you`re more than likely storing up problems for a later date, not to mention the possibility of damage to your entire system, should something go wrong. Sadly all to often this is what happens.
Take a look
HERE at this excellent article on psu`s.
Not only does it give recommendations on which psu`s are good, but more importantly which psu`s to avoid.
Regards Howard