If both tyres are gone, then it is not even worth 15k. Tyre replacement will cost arourn Rs. 3000 at the minimum, so the asking price can be 12k only. If the bike is maintained well, i mean paint look and smooth engine then 15k can be considered. Is it 4 speed or 5 speed, RX135 or RXG?
And also the price depends on where in India you are located. Metros will be costly, small towns will be cheap. In Bangalore you will have to spend 20k minimum to get a battered rx135. It makes sense to buy bikes from smaller towns ofcourse from the same state(so that you need not register the bike again).