Hey Laxmi,
There is a bus from leh to delhi Bro ... And thats the cheapest you can get to travel from leh to Delhi ....
HRTC has started a bus service and here is the link from Tripoto wanderer which h will help you in all means ....
Please check this link and decide about how and when to book tickets and stuff ...
https://www.google.com/amp/s/www.tripoto.com/trip/new-delhi-to-leh-ladakh-bus-service-by-hrtc.amp
It takes time but will relish your experience for your life ...
Happy Travelling Bro!!!