The bistros are normally the best value for money, also a lot of places on the streets to buy snacks. Souvenirs around the tourist attractions are generally more expensive look for street vendors ans smaller shops in the streets. almost 6 years ago
For authentic food I would suggest checking out some of the city's brasseries~ try to stay away from the areas that have a high volume of tourist traffic and you'll do better price-wise. Take advantage of the "formule" menus. You can usually get two or 3 courses for cheaper than one of the main courses on the a la carte menus... or pick up some good bread, some cheese and charcuterie from an epicerie or even a monoprix (buy poilane bread there). Bakeries are always a good bet for a cheap snack or breakfast, though beware: not all croissants are created equal!
For souvenirs, I had the best luck finding things at one of the city's flea markets. I like the one on the weekend at Porte de Vanves best. You can find things like antique postcards, photos and cast iron keys, handmade lace gloves, lots of things that make great mementos and keepsakes... don't be afraid to barter on the prices.
I also think food items make great gifts for people back home: mustard, oils, chocolate, salted butter caramel...
Have a great trip! almost 6 years ago