That’s it, just say NO.
If the work is sufficiently complex to require special skills (yours) then it’s of sufficient value to the client to get paid.
I recently went through this dance. A prospect got in touch via email (referral from various sources), then we did some phone tag, several conference calls, a ream or more of additional email, and then a meeting was arranged.
For me, it was a two-hour drive early in the morning (I’m a night owl) to a breakfast meeting in a diner. Got there, and things started downhill almost immediately. The client principal wasn’t in attendance even though she would have to approve any ‘deal.’ The talk quickly turned to my doing this for ‘exposure’ (sorry, No); then well “you do the design and if we like the design then you can bid on the job and if you’re the winner you get paid after the job is all done.”
Not playing that game… time to leave. They wanted a ‘ball-park’ figure; I gave them one, and then added that it would of necessity be much higher should they return in a few months – disgruntled people are much more difficult clients. When they told me it wasn’t likely, I wished them success – with all those other consultants they’d tracked down for this sort of work.
My exit was made in silence, at least from that group. I expect they’ll be back, and my answer will, for them, always be NO.
End result – I think I’m going to have to start charging for prospect meetings, especially if the prospect isn’t used to dealing with custom software.