I've never had a MPA complain about wanting to end things early. If I paid for 30 mins and wanted to leave at 20 minutes, zero complaints. ever. Usually the problems arise when you pay for 30minutes and it drags on to 40 minutes.
So if saving time is what you want, then I don't believe there is a single MPA that would say you can't leave early.
But if you just want a 15 minute hj to save money, then that's something else. You will likely have better luck at slower places. I haven't looked so I am not aware of any places that has any sort of focus on high volume, low transaction fee hj. I guess.. what's an acceptable price point? I guess you could just pay the 30 min door fee and $10 and see if the MPA is willing to accept it. might have to compromise on stuff like nudity though, also keep in mind some things like massage oil, especially good ones, have gone up a lot in price. Like $60 or more per gallon. so that adds to their overhead cost if they are going to use $5 of oil on you.