I've been to London THREE times and yes I found Paris to be MUCH more expensive. ESP. for what you get! At least in London you get a full plate of food for what you pay! Not a piece of bread with a slice of ham and a sliver of cheese called a sandwich! LOL
Clarife, you can try the bars in "Oberkampf" or the "Butte aux Cailles" districts. These are nice, not too touristy and the prices are reasonables.
Avoid large "cafés" on the main avenues and boulevards. These are expensive, the food they serve is not very good, and waiters are rude and frequently try to cheat tourists.
Have a nice time!