I´m collecting only PC adventure games (point and click).
The prices really vary, a lot.
It depends on so many things like the overall quality of the item, the version, the rarity, completeness and so on.
And then it´s always a point of time thing that sets the current price.
I´ve some games that are worth double the price I´ve paid for 1 or 2 years ago.
Other have droped to certain reasons.
So it´s really hard to tell and nearly impossible to create a guide for it.