This would need to be implemented as a $30 membership that gives a $1 discount for every check-in over a 30 day period for the next month's payment, or better yet, $60 with a $2 daily discount because memberships below $30 aren't really enough to run a gym.

It would be better to give credit that can be exchanged for things. Better yet, become the company that sets up turnkey reward programs for active gym members.

or you could overcharge them, say $60/mo, then say to get the 30 back they have to come in daily.

The problem is at $50/month+ the service expectations become much more expensive to provide -- even though a dedicated member could get it back into the 30s.

Edit: what I mean is that if price ranges from $30-60 the average cost will be a lot nearer that range that's perceived as upscale gym. Whereas ranging from $0-60 would at least appear to be nearer the ordinary $30/month average. It would entirely depend on usage, obviously.

I recently heard of GymPact, they seem to be doing something similar.

That's cool, but if everyone works out, they run out of money. I realize that is unlikely, but it is a system just asking to be gamed.