Scott blue Int'l collections at regular auctions (not
ebay) generally go for around 1-3 cents per stamp, with the expectation that there won't be anything above $100 catalog or so (first off, because blue Internationals don't have many high-value spaces, and secondly because the best stamps/sets may have been picked.) Maybe a little more if it's a complete set of albums in nice condition, or if the albums are very full.
I'd use that as guidance with
ebay collections, as well; you can generally do a lot better than $150 for 2,000 different stamps (as in the first link).