Just got back from six months in cooking school there. Two of my favorites were Chez L'Ami Jean (make reservations a couple weeks ahead if you can) and a wine bar called La Cloche des Halles. L'Ami Jean is very boistrous and folksy and serves dishes from the Basque region. It's a blast; go for one of the game meats from the special menu or just order something from the prix-fixe. Can't go wrong!
And I liked Cloche des Halles for a nice charcuterie plate and a few glasses of very reasonably priced wine on Saturday afternoons. It's cramped and jolly and cheap, and located near all the wonderful cooking shops of Paris so you can browse after you eat.
And I know you said "reasonably priced", but if you want to splash out, try L'Astrance for lunch. It's certainly pricey but still a good deal for the quality (and quantity!) of food you will receive.
Chez L'Ami Jean
27 rue Malar, 7th arr.
01-47-05-86-89
La Cloche des Halles
28, rue Coquilière. 1st arr.
01 42 36 93 89
L'Astrance
4 rue Beethoven, 16e
01-40-50-84-40