How does it work for Iphones pay as you go?

My friend's Iphone contract ends soon and he was thinking of unlocking his Iphone to put a pay as you go sim card. Does that work? Will it be the same price plan as with any other phone? He is currently with Vodafone and its an Iphone 4.
We believe pay as you go is just so much cheaper than contracts.

Yes it will work and yes same as any other phone it only depends on what package of pay as you you pick it doesn't make a Differnce using a micro sim to a normal one there's no price difference