Asking for a detailed quote (where the one time effort taken to create the quote is significant compared to the value of the contract) probably isn't reasonable

Asking for a detailed quote (where the one time effort taken to create the quote is significant compared to the value of the contract) probably isn't reasonable. If you ask 500 freelancers to spend a day making a quote for a months work, you just cost many times your contract value. A suggestion: A better way is to describe the contract, and provide a brief 'pre-qualification questionnaire' where a candidate proves that it is worth both your time proceeding. After that, you invite bids from the top say 6 applicants, and work with them to get the best bid possible.