Hey,
For small motors up to about 1 Amp, an LM293D will suffice. It's a small 16 pin DIP IC with 2 full H-Bridges onboard. I don't know exactly how much it is but it definitely won't be higher then the region of a few pounds or dollars.
As for building one, all you need are some transistors or MOSFETS, so the price will be quite low. You will probably also be able to make a higher current H-Bridge for the same amount of money spent on a pre-made one.
So really you have to consider the current of your motors and whether you wish to build your own rather than buy one.
-HyperNerd