Lol, I just paid £20 for my MOT today 
Go to a council garage, they're less inclined to sell you stuff.
I disagree 100% with that, as, dvla set a price, say £54, garages then have to pay a certian amount to dvla for how many ever mot slots they want..( i dont know how much it is, but am sure £20 dont cover theres...)
Caz, I think you missed a crucial point, which Ben made. A lot of local councils (local governments) have their own (often very large) workshops to maintain their fleets of vehicles - which also need MoTs. Also, being as it is those same councils who run the licencing schemes for the Hackney Carriages and Private Hire taxis, which MUST have an MoT every year from their 1st b.day and not 3rd like private cars - most councils officially subsidise their own MoT stations (to 'look after' local businesses, which taxis effectively are). But by subsidising to taxis, they also have to offer the same subsidy to any private motorist too. OK, they may demand you actually come from the actual borough or local authority area - but say you lived in Nottingham, you could go to the Nottingham council workshops for a cheap MoT (providing Nottingham council do offer the subsidy).
I reckon for certain places, they do it on purpose, do it cheaper TO find things wrong, to make thier monies up. Plus, theyre maybe not as thorough as a main dealer for example.. as we had a car in not long ago that had an mot done at a garage garage and failed on an item, so they brought it to us for work, we did that item, but also did a quick check of car ourselves (duty of care) and found all sorts wrong with it.. the vehicle was no way road legal..
Hmmmmmm . . . MoT stations, just like 'garages' - weather they are independents or part of a franchised dealer - can all vary. Some can be good, some can make work up. I have seen opposite ends of the spectrum at both main dealer MoTs and independent MoTs.
