My rule of thumb has always been 3 hours per dollar spent, for games as well as DLC. The very best games and DLC alike have given me a cost per hour that my grandmother could have afforded when she was a child in the 1930s going to the movies for a nickel.
I have never gone for games that are finite. 95% of my collection is open-ended games like city builders, trade sims, and other extremely open-ended game styles. Had a Sims 2 neighborhood that lived the entire life of my last PC, fully four years. It was astoundingly well fleshed-out by the time I was done.