I found it hard to determine what to pay for a new sled, so I priced all the dealers in my area (and sometimes slightly out of it as well). This at least gave me an idea. I also learned that $500 isn't worth leaving a good dealer for some unknown. I just bought a sled out of state and got it for just over dealer price plus NO taxes. Nobody in Colorado could come close.
As far as depreciation, there is no magic formula. You could go to KBB and get a price, but it won't hold water against variables like the current economy, time of year, condition, mods, miles, etc. If you want to sell something and you are not sure, find other sleds like it on Craigslist, SnoWest and other 4M's to get a baseline. If it won't sell, you need to lower the price.