I believe it was shortly after 9/11 happened that training costs skyrocketed. I don't know if it was due to insurance requirements or just greedy flight schools.
I don't see a ton of flight schools making a lot of money. Margins are tight these days.
Besides insurance and general inflation, 100LL prices have skyrocketed. Today a student in a 172 might burn $50/hr just in gas. Back in the 90s, you can fly a nice twin for like $45/hr wet (all inclusive) and the school can still make money. It's a different world now.