I agree with Shawn & Stephen – if a trainer has built up the clientele, than it may be more worthwhile to pay a flat monthly rental fee. If they only have a few clients or is a part-time trainer, a commission may make more sense.
The last time IDEA did their compensation survey, the found the commission to be a split between 50/50 or 60/40 with 50-60% for the trainer, 40-50% for the studio.
As for a rental fee, I’m not sure if there is an industry standard for this. You may be able to estimate a figure based on the number of clients full-time trainers see per month, the price they charge, and base the rental fee on the commission split you would get from these total sessions.