If we are talking about full size equines. It depends on many factors but for most around here, it starts at $500 a month. If it's for breaking (a 3 month mininium) with show horses ( 1 year mininium). Then there are extras: vet, farrier, show prep, trailoring, supplies..etc. Top notch trainer start around $1,000.
It truly depends on the trainer. Well known trainers may well charge much more just because they are well known BUT you can find excellent trainers that aren't "breed" reknown and get the same results and then you luck in like I have - finding a trainer who is well respected, caring towards his trainees (horses) for less than what I would pay someone who has much less experience of exposure in the breed. I would highly suggest that you research who you are interested in perhaps using and talk to former clients of that trainer to get a good over-all picture of what the trainer has accomplished in the past and how the animal was treated and were ther expected results achieved.