Allow me to rebut as a vendor
If you need costs to be predictable and upfront, make your requirements and needs 100% crystal clear, rather than making them as vague as possible in the RFP and then trying to scope in the details at a later stage when you think the price has been set.
If you need good answers to your questions, and good submissions for the RFP, don't set ridiculous deadlines to submit the RFP and related answers to questions. Also, be prepared to interact with vendors in a timely fashion as needed in order to clarify questions. And lastly on this topic: ask the right questions.
And lastly, and this can't be said enough : cheap is not always cheaper. ;)