What is the going rate for a certified trainer per hour for one on one instruction?

It depends a lot on your area, whether they are in a gym or outside of the gym, whether it's a promotional rate or the standard rate, how experienced the trainer is, etc. I've seen rates from around $25 - $200 an hour, with most falling in the 50-100 range.

I currently pay $64 an hour (12 sessions for $768), but have paid as low as $55 for an hour. Also, the mores sessions you buy the lower the hourly rate is.