It's not unfair to charge for a proposal (and the process that generates it), but it's unusual, and it creates a large risk that the client has to shoulder (if I don't like your proposal, then what? The proposal is most of what I have to go on for how good a fit you are in the first place!)

I wouldn't do business with a contractor that demanded payment for proposals. I say this as the operator of a business in which proposals are seriously expensive to generate. I wrote a large proposal last week after a series of phone meetings, and the proposal didn't generate any business. Oh well! That's life in the big leagues.