Below is a phrase with just thirty-five (35) words that should be placed at the end of every bid/proposal you send out to prospects. By using this phrase, you will be asserting a measure of professional control over the prospect that will establish the fact that you need to be treated with respect. I suggest that you give them between three weeks and five weeks to finalize your bid.
"We will be pleased to hold a tentative reservation for your event date, and guarantee the above prices until (Deadline DATE). If not confirmed with your deposit by this date, availability and prices will need to be renegotiated."