If you look at an invoice its like 45-60 for the AI but that doesnt count the board, palpations, drugs, hormones, etc. I tell people 300-600 to get a mare in foal with cooled shipped semen on one cycle bearing no other issues like culture and treatments and lavages.
My vet charges $350. There is also a cost of shipping which can be anywhere from $350 to 450. That is if you catch the mare in the first try, if not you start all over again. I have had a vet charge $700 for the exact same services. That vet just would not quit sending me bills! I think you can ask them in advance. I would certainly use a vet that knows how to AI and does it regularly or it can get expensive in a hurry.