The following are the best eating joints for those on a budget.
south indian
Banana Leaf near Lake Market Stand, South Kolkata
Rolls and Kathi Kabas
Ice Berg//Badshah near Gariahat/Golpark
Punjabi
Hotel Azad Hind on Ballygunj Circular Road (spl. egg tarka, chicken bharta

hyeah
Mughlai
Shiraz (park circus) - known for special Biriyanai (spl type of Awadhi biriyani with Calcutta's own flavor)
Bedwin - Gariahat, south Calcutta
Royal (north Calcutta) - chicken chapps are lip smackign good
Bengali Authentic
6 ballygunje place near fari, south Calcutta
Continental
Amber near esplanade, Calcutta
Last but not the least Park Street in Calcutta houses most kinds of restaurants that fit people on a budget. Sizzlers from Peter Cat, Chelo Kabab from Magnolia, A La Kiev etc are great and cost hardly like 100-150 a plate. Great VFM.
Statutory Warning: Once you are used to the impressive yet relatively inexpensive Calcutta food - you will be hard pressed to find similar quality at a price elsewhere.