Winners pay the auction price, not their bid. Set your max bid to what Marinade stake earns you, and you win stake only in epochs where it pays. Every other epoch you sit out and pay nothing.
The SOL in your bond is delegated to your validator. It earns the same rewards as any other stake on you while it guarantees your bids.
Withdrawing takes a few epochs, so you can’t pull it out the same day. Fund it with SOL you won’t need on short notice.
Your bond only pays out at the auction price, and only in epochs you win. With your bid at break-even, a won epoch is never a losing one.
See your break-even, your last 30 auctions and the exact commands.