Most of us are here discussing about the new Micromax Canvas HD 4, and some considering to buy that. My question is what do you do with your old phone when you upgrade your smartphone?
I have LG Optimus Black P970, almost 2 years old now and had bought it at 18k. Am planning to buy a new smartphone now in 20k range. How much could I expect to get for my old LG phone? It is still in very good condition.

You have two question..
1. Which phone to buy in 20K range
2. What should price of your USED LG P970

Well, for first question , I suggest you stretch your budget a little and go for Xperia SP around 23K now , a very good phone or Google Nexus 4 .

But I think your phone is fine unless you need a bigger display.

I agree with Savera's suggestion. But I'll address to your query on "what do you do with your old phone?"
When we generally upgrade our mobile phones, there is no buy-back on the old phones as is the case in some European countries. What we do with old phone is that we sell it or just dispose it. Its fine to sell it.
But concern is with disposing the old phone. We are not concerned about the e-waste that is being generated, and we should not just simply dispose off our electronic stuff straightaway in garbage but through some e-waste handlers.
I really do not know how many of us are concerned about this point.