what would be estimated price of my car if i want to sell it now.

well i have hyundai i20 spotz 2010 july. every thing working properly. the body has some scratches for normal everyday use... battery replaced... i would like to know what price should i ask if i want to sell it now. since it is a petrol version and its getting heavy on me . i want to take a diesel car and get rid of this. someone told me i should not expect more than 2.5L for the car.. but i visited olx and other sites. and saw similar to my cars with same km done for 3.70L. it was not a single user asking but almost every one was asking price very close to it.. at about 3.5-3.7L...

so please suggest me how much should i expect for the car.

btw kms done is 21000Kms
thanks need sugestions
thanks