If you are in a location that commonly uses it, selecting "sold" on eBay and looking at the last couple of weeks is a good indicator. It can be a bit difficult in regard to "local" sales, but should give a good starting point. In my own experience using sites like Craigslist, LetGo, Marketplace and such tend to have lower and often very volatile swings in sold price, if you can even determine such.
Personally, I feel ~150 is probably a good spot to start with eyes on OBO.