Somewhere on here theres a long informative thread about PPI's, what they can find out for you, what they can't find out for you. A leak down test though is a must have though. I was quoted around $350 from Beurkle Acura in MN, but I went with an NSXPrime Member that knows his way around the car and has a shop for much less.
The first thing most members will do is post in the regional forums looking for someone that can do a PPI or at least put eyes on the vehicle and report its general condition before ordering a PPI. IMO, you only want to order a PPI on a vehicle you are VERY sure you will buy depending on the outcome of the PPI, otherwise it could get expensive.
Theres also an excellent link to a very in depth NSX buyers guide but I no longer have it. =) Someone will see this thread and hopefully post it.