http://www.mybluefin.co.uk/Superchips quote £300 for (up to) a 10bhp and 10lb/ft power increase.
It's a simple plug-and-play operation, takes about 15-20 minutes all in, totally DIY. You can only use it on your car, and if you ever want to use it on another car for any reason you need it reset by the manufacturer. This usually costs a bomb, but can be discounted or free if you have a legit reason for having it reset.
Hope that helps!
I've not used one for the ST so can't say what practical difference it makes to that car, but I have one on my Focus RS and it's great (has a turbo though, so the power increase is a lot more.)
-Alex