I've used a personal trainer for the past two years and consider it to be insurance. We go twice a week for 1/2 hour sessions (I think that the half hour gives you enough time to get something done). It provides the motivation we need to get to the gym, period. Because of the trainer, we've lost 60 lbs EACH and have kept it off for almost a year.
The cost is $30 each, so a total of $60 per session, 2x a week.
I can assure you that there is nothing else in our budget that looks like this, but it is simply a commitment we made to get and stay healthy. Also, my employer pays half, so our out of pocket is $240 a month. The trainer also pays our gym membership.
We lucked into a great (male) trainer through a friend's referral. I think the key to finding value is knowing just what sort of trainer you need. We don't need to know any fancy routines, we just need someone who we HAVE to be accountable to every week. I would like to think we could be motivated through (cheaper) personal accountability, and if that were the case, I'd do it in a minute, but this is who we are.
I consider it to be the most important money we spend every month, seriously.