They're relatively cheap, so I don't feel the cost when I buy piecemeal. Buying together is a bit different. The thing is, the production value is much higher. Seeing Isabella speaking Spanish or Pachacuti speaking Quechua or Harald speaking Danish is quite good (and the animation is usually beautiful). In the end, it's more variety. I'd argue the value per Civ is higher, but the value per dollar is less if that makes sense. In other words, you would certainly get more content with expansion packs, but I think each civ in the expansion back has more substance to it. Each one plays very differently and it's just cool seeing them in game.