Not 100% positive, but I think $200? It might be listed in the contract fine print online.
Okay, I found this under renting with a debit card "Renters without a ticketed return travel itinerary will need to provide a credit card with sufficient funds to cover the cost of the rental plus $200. Debit cards are only accepted at the time of rental if accompanied by a ticketed return travel itinerary."
So from that, it looks like they don't put a hold on a credit card rental, but you need to have a return ticket if renting with a debit.