New to PC mods. How often can I expect a GREAT one? Thread

Stuff like Portal Pro or Third Age: Total War, for example. I've learned to use a discerning eye for both indie and AAA games, and I'm sure I will do the same for mods. In a typical year, are there enough quality mods I'll be satisfied? Mods that make me say, "Damn I would have bought this at full price."