Depends on a lot of different things in my experience.
First off, unless it's a prestigious event that will attract a lot of well heeled visitors with cash to burn, and most are not, then you are most likely to shift only the usual small stuff, light pulls, keyrings, mushrooms, fruit, smaller bowls, pens etc.
Most people seem to go to craft fairs for entertainment and a day out rather than to spend much money, but they might be tempted with an impulse buy under a fiver or even if you are lucky splash for a smaller bowl or similar at ten to twenty quid, but most sales in my experience are under a fiver.
The exception I've found to that is near Xmas where people are desperate for ideas and something different for presents and are resigned to spending a bit of dosh and in those circumstances I have been able to shift more stuff in the ten to twenty price range but not very often anything more expensive.
Bit sad but true that most people aren't willing to value the craftsman time and seem reluctant to pay say thirty quid for a large salad bowl even though in the posh kitchen shops in London the large ones start well in excess of £100, but of course that covers retail costs etc.
Even so, to make a large bowl, from sourcing the raw material, breaking it down and converting to blanks, turning and finishing etc takes a fair bit of time and even if you only priced your labour at say McDonald's rates of whatever it is now say £6.50 per hour you would probably have spent a good few hours start to finish even before you add in overheads, travelling and stall fees let alone a profit margin, so thirty or even forty quid may well not even cover your basic costs.
So I think the opportunity is at the bottom end and the top end of the market (the really creative fine and artistic stuff that the likes of Stuart Mortimer etc do that they can rightly charge many hundreds of pounds for), but the middle ground is pretty difficult.
Guess many hobbyists like myself end up selling surplus stock just to plough some money back into the hobby and making a profit or even covering costs isn't the objective, but there is an argument that this in itself undermines the pro's who need to charge more to keep going.
Cheers, Paul