The maximum fee that a testing station can charge for an MOT is �50.35:
http://www.direct.gov.uk/Motoring/OwningAVehic le/Mot/MotArticles/fs/en?CONTENT_ID=4022514&ch k=3fdPzr
Most garages charge the maximum but some offer a discounted figure.
As has been suggested, it can be a good idea to find a garage which offers a free re-test. However, if you know a good independent local garage (irrespective of whether it's a test centre), it's sometimes simplest to book your car in with them and tell that that you want it to get through its MOT test.
That's what I did with my car recently. I simply told the garage, "I'll drop it in Friday morning. It needs a new wheel bearing and a bulb. Fix anything else which needs doing and get it tested for me". I collected it a few hours after I dropped it off. Everything was fixed and it had been tested at a centre about 5 miles down the road from my garage.
Doing it that way meant that I knew that it would pass the test because my favourite garage had already checked it over before taking it to the test centre. It also meant that I knew that all the work would be done at a reasonable price. (If a car fails a test, it's always tempting to let the test centre do the work - this can prove to be expensive!).
Chris