I am not sure how many days or Where you planning to , but i can suggest you this.
If planning to travel to City.
Use Trains ( Esp City express) Quite economical and Fast too
Have street Foods. Planning on Trek Borrow a tent use public transport
The only expense would be permissions. and food ;)
Use public transport, if train then book general compartment.
Always have food from dhabhas( road side stalls) and opt for vegetarian.
Visit Kerala and places in Kerala like munnar and wayanad, most part of these places are accessible by local transport.
Reach Kochi by train and then take ferry to fort Kochi for Rs. 7. Buses to munnar only costs less than 150.
Use KSRTC buses for Kerala transport.
Instead of renting a tent, sleep in AC waiting ros in railway stations at cheap cost ( always remember to take platform ticket also).