Prices vary *widely*, and I mean REALLY vary depending on location - they'll charge whatever they think they can get. One of the first questions you'll be asked is, "soooooo, what do you do for a living?" as part of the casual conversation, lol.
Think of them like car salesman - you can usually negotiate them down a LOT from the initial quote.
I've heard of everything from $100 to $500 per session. My area averages around $450 per session (called around 5 places), but that's VERY high from what I hear. It's just a fluke of my geography.
They usually press you to buy a "block" of six sessions at once, at a reduced per-session price. You can usually get the reduced price anyway though, paying as you go, if you whine and complain like I did

Most will suggest six sessions are needed, spread roughly one month apart. Many TSs have told me they've needed 8-20 sessions to get nearly totally clear though. 90% of my beard fell out 11 days after the first session, and it's stayed that way so far through session #4. Between sessions, the last hairs to be zapped fall out while new small patches appear, so it stays at that 90% overall gone level.