In the Civ series, generally speaking, expansions have improved on the existing game. That said, the same does not hold true across all game franchises. I can think of other game franchises where the expansions did not really give bang for your buck. I think this is why DLC has become popular. It's cheaper to produce, provides a steady stream of income to the company, and is priced at a low enough point to attract impulse buyers. By contrast, a $20-30 expansion that doesn't live up to your expectations might end up hurting the franchise in the long run (and yes, there will be a franchise if there's an actual expansion).