Your travel budget goes much further when you're able to account for every dollar. Look for stores with clearly labelled prices to avoid being charged a hefty "tourist fee" on top of what locals would pay.
Image from davidrosenphotography.
Rick Steves shares an example of how this double standard comes into play when prices aren't displayed:
Travellers are routinely ripped off by market merchants in tourist centres. Find places that print the prices. Assume any market with no printed prices has a double price standard: one for locals and a more expensive one for tourists. I'll never forget a friend of mine who bought two bananas for our London picnic. He grabbed the fruit, held out a handful of change, and said, "How much?" The merchant took the equivalent of about $US4.
I had a similar experience in Kyoto. I wanted to fight the humidity with some refreshing watermelon, but since it is imported from outside the country, it was much more expensive than I was used to. Luckily, the store I bought it from had the price clearly displayed so I was ready for the bill. Make sure you buy from vendors that do the same and you won't go over your travel budget by accident.
Tips on Eating in Europe [Rick Steves]