I sympathize.

As others have said, there is a difference between submitting a proposal and delivering an analysis and specification. I had a few of these where I was assured after spending a couple hours with the client that I was what they wanted, but after I delivered the detailed "proposal" with a cost estimate, they went shopping for somebody to do the work.

That is why I have for the last ten years made it clear from the beginning that I will be charging for my time preparing a detailed analysis, specification and estimate. If they insist on a free proposal, I give them a cursory analysis and propopose a time and cost to create the detailed analysis, specification and estimate as the first stage of the project. I make it clear to my potential clients that they can then take that document and shop around if they wish with no ill feelings on my part.

I've lost a few potential clients over this policy, but not many. (I'm not sure I would want to work for them anyway. That's not sour grapes. It's a question of mutual trust and fairness.) And I've almost always gotten the follow-on work to complete the project. The times when I didn't get it, the client decided it was too expensive and went with their plan B. The way I present it, most of my clients appreciate the opportunity to review the detailed specification and tweak it before I begin work.

...Jim