That's a tough one. I voted I'd spend 50% more, but really I need a 'depends' option.
Depends on the quality. Depends on how unique, interesting, hard to find an item is. Depends on my mental budget/what else I've spent money on that week. Depends on the FM. Living where I do, there are A LOT of FM that cater to tourists, and I avoid these like the plague, being over priced and often not even local produce (which is lunacy to me). Typically, I'll pay more if the quality is at least as good, but more likely to if quality is better. But some things I will spend a lot more on. Local honey being the one that comes to mind because the guy I buy it from is literally two miles from my house and I like to support this dude who leaves his honey out in a closet on his porch, trusting people to leave money in exchange for the honey. That guy is awesome.
And I don't judge the quality of the produce necessarily by appearance. A lot of heirlooms, especially tomatoes and apples, look way weirder than the nice, perfect supermarket products, but with superior flavor. On the other hand, I can grow those awesome tomatoes. So do I pay a premium for them? Probably not. But things I can't grow, definitely.
Because of how much we grow, going to the farmer's market is either for things like a 50lb bag of potatoes ($8 at this one place and whoa nelly, even when I grow them, I can't grow them that cheap), 150lbs of tomatoes (.50/lb at a pick your own place) or specialty items. So it's more of a discretionary purchase for anything not in bulk.