We have big difference in prices from $35 with backyard trainer (which is waste of money) to $75 with the dressage trainer (frankly that's too expensive IMO, but...). I usually try to find "the golden medium".
Some trainers in area have different prices for 30 - 45 - 60 mins. I really dislike such places (was burned twice) because even you say you want 45 mins (c'mon, not everyone is rich as Bill Gates :) ) they keep going and then charge you for 60 mins (or even 1.5 hours in one case). Well, I probably should just break it at 45 mins by telling them to stop, but I never feel very comfortable doing that. :(