This is not my bid but a random one I saw. But this kind of proves my point. The #5 bidder,bids for 1 dec and got 11 dec/day for the card which does not go to your goal of "Auto is cheaper than manual".
What I assume you can do is you can do an escrow like the normal renting. If I bid for 10 dec, that 10 dec will be removed from my account until the bid is filled or I cancelled. This way, the auto renter system will not be able to provide or fill any higher than on that escrow. If the bid was for 10 dec and the cheapest is 12 dec, the amount is insufficient, therefore it cannot fulfill the bid until a cheaper one does come.
A follow up since this is important.
This is not my bid but a random one I saw. But this kind of proves my point. The #5 bidder,bids for 1 dec and got 11 dec/day for the card which does not go to your goal of "Auto is cheaper than manual".
What I assume you can do is you can do an escrow like the normal renting. If I bid for 10 dec, that 10 dec will be removed from my account until the bid is filled or I cancelled. This way, the auto renter system will not be able to provide or fill any higher than on that escrow. If the bid was for 10 dec and the cheapest is 12 dec, the amount is insufficient, therefore it cannot fulfill the bid until a cheaper one does come.