YC's advice around these kind of talks is that they're a colossal waste of time. If you're interested in an acquihire situation then there may be some value in meeting, but otherwise the chances of something meaningful coming from this is very low.
Remember, for these M&A guys, meeting people like you is there job. For them having these meetings is a productive use of their time. It costs them nothing, and they only gain more insight into the marketplace.
What do you get out of these meetings? They're not going to tell you anything special/secret about their product/market.
What I've seen some people do is have an initial call/meeting (don't travel for it) for about an hour to feel things out. If it seems like there's genuine interest then have there be a "break up" penalty. i.e. if things don't work out they pay you $5-10K.
That way you get something for your time if it doesn't work out, and if they're not serious they won't agree to it.