Can't give you exact rates but it seemed reasonable when I was there 2 years ago, compared to UK.
Best value and very nice too is Franco's sandwiches and pizza, and you can buy takeouts including wine - they even pulled the cork for us cos we didn't have a corkscrew. Franco's is near the railway station on the main coast road, in the direction going out of town.
You can also get the train straight to Pompeii or even (Naples if you fancy a fight) from there and it costs very little.