Thermae is pricey. I picked out a really attractive, 25-year old girl. She quoted me 3000 baht ($87) for 1 hour, 5000 baht ($146) for 2 hours, and 8000 baht ($233) for overnight stay (till 6 am). I didn't have a hotel nearby, so we went upstairs to the Ruamchitt and I paid 700 baht ($20) to rent a short-stay room. I paid her for 2 hours: 1st hour was fun and activities, 2nd hour she asked to go somewhere to eat. Overall, I spent 5780 baht ($168) for 1 hour of smash at a pretty decent hotel room, 1 hour of walking and holding hands to a nearby eatery, and 1 expensive glass of coke (entry fee into Thermae for guys is 1 drink. I got a coke for 80 baht, which is pretty expensive for just a coke).
Yes, I realize that $168 is cheap compared to what you'd pay in the sexually-repressed U.S. of A, but in comparison, I was able to have just as much fun with a slightly older (mid-30s) Thai girl who works at one of the many massage parlors on Sukhumvit 24/1 Alley for only 2000 baht ($58).
So if you're still interested in Thermae, I'd recommend booking your stay near Thermae (there's no shortage of hotels nearby) and doing only 1 hour with the girl. Trust me, with the plethora of choices inside the bar, it's hard to justify spending more than 1 hour on the same girl, unless you happen to form a connection with her.