That depends on how good the class and the facilities are. It costs money to run schools, so for a good school a 100$ doesnt seem unreasonable, what are you getting for it, how many lessons, workouts, do they have a gym, showers etc...
got a couple guys i know that take it.. Come home after the first week all sore, bruised, scrapes, but well worth it.. now encompass that with muay thai, you'll be kick ass.