A few months back I made a spreadsheet of all my games. Then made columns for Rating, Plays, and Price (approximate if I couldn't remember or received it in a trade and also added together expansion costs into a single listing).
Then I came up with an equation to determine how much Value I have gotten out of each game based on the above data. But I also weighted the equation toward Rating.
Value = (Rating+(Plays/5))-((Price/Plays)/2)
Dividing Price into Plays tells me the cost per Play and then dividing that by 2 is again just for weighting the overall Value towards my Rating for the game.
I'm sure there is other ways to determine how much Value has been derived from a game, this is just mine. For reference, of the top 20 games on the list, I would only consider 13 of them to be among my favorites.
I could not get a proper spreadsheet table pasted in here so here is a link to the actual Google Sheet: BG Ratings
[EDIT] Here is the updated spreadsheet that incorporates playing time into the formula: BG Ratings w/Play Time
The new formula is:
Value = ((Rating*10)+((Plays*Playing Time)/5))-((Price/Plays)/2)
This new formula created a lot of changes at the top and the results are something I am actually more comfortable with than the first iteration.