How much is my MacBook PRO worth a.t.m.

As a starting point, you can check Mac2Sell, eBay (including completed sales), and Amazon for prices of similar models, to get an idea of what a reasonable price may be. Then adjust the price to account for condition, configuration, remaining AppleCare coverage (if any), etc. You can also check the Apple refurbished store to see what similar models are selling for.

I would have to say that while prices you see on eBay and http://www.mac2sell.net are sensible as a starting point, a lot depends on your location too. I recently sold an old MBP of mine and while I listed it at a price similar to what mac2sell suggested, I ended up selling it for about 20% less, to a local buyer who paid cash. I would not have this option if I did not live in a large city.

eBay prices could be a little higher, but you would lose about 15-20% to eBay+Paypal fees, plus there is a higher potential for buyer fraud which could give you anything from a major hassle to loss of whatever money you made on the transaction, so I try to avoid it for selling high priced items if possible.