I think farmer's markets can be a mixed bag really. When I lived in Reading there was a brilliant one near us, but even then some stuff was very expensive for what you got and other stuff was well worth it. After having a good nose around the first few times I worked out what was good value, e.g. there was an apple stall that I loved, that sold huge sacks of delicious local apples for about £3. I'm a bit of an apple fiend (how rock'n'roll of me) so that worked out much cheaper and more pleasant than buying supermarket polybags. Meat was also very good from there, good value and good provenance. Then there was the stuff I'd class more as treats, like tasty but pricey locally brewed beer or fresh apple juice.
I'd say it's worth having a look around anyway. No clue about the spuds though, sorry!
Go along for a nosey round, definitely. I like to get mince and sausages as they are not that much more expensive than supermarket (ok they are more than value mince and you won't get 8 sausages for 16p like you do in Asda) and they really do taste a lot nicer. Fruit and veg are variable. The stuff that's the most expensive are things like handmade bread and cakes, locally brewed beer and wine, that type of thing (though lovely for treats). THey are good to take DCs to as well as there are usually lots of things to taste.