Give a few places a call, it's pretty much fast fit so you don't need a top end garage. It depends on your area really as to whether those prices are good, I always do discs/pads myself since it's so simple and quick. Being a Mini garage they will probably use original equipment pads/discs where as some places may skimp (or offer better parts).
In terms of parts, for the Mini One:
Pads (front):
Part Number : MDB2162
Price : £26.64 + v.a.t (axle set)
Discs:
Part Number : SBG3661
Price : £15.44 + v.a.t
(Individually Priced)
Rear pads:
Part Number : MDB2163
Price : £21.21 + v.a.t
(Axle Set)
Which will give you an idea of what labour they're charging you.
I had a quick look for some basic pricing on the net and didn't find anything immediately. What mileage has your mini got on it? It will have wear indicators which should tell you when it's worn. You shouldn't rely on that but I would be slightly dubious that they need doing and the wear indicator hasn't told you to be honest. Depends on your mileage though.
Anyway, to answer your question; they're not silly prices.