I've never really had to do a holding fee (that I can remember, I have a terrible memory lol) but I guess if I did, I would just charge them my holiday rate which is $10 less per day.

I don't think it's unreasonable to expect full pay though. It's not your fault that they don't need care when your spot opens up. It's a chance they can decide to take but you shouldn't have to loose out on money