Not so much if you watch for deals-
At Christmas-time, Kmart clearenced out Spyro's Adventure, so I got the 3DS and PS3 versions for $19.99 each. Some Best Buy stores had a 50% off red stickered games back in January- For $40 I bought a copy of the first game bundled with two figure packs (1 3 figures, one 1 figure and the pirate ship). Between the two Spyro's adventure game and the two figure packs I still didn't have an Air type so I did have to spend $10 for one of those.
I also just recently picked up Skylander's Giants Portal Owner's kits for the 3DS for $30 each.
So, for 5 games, 3 console specific portals, at least one figure of each element type plus extras, it ended up costing me $150. Don't forget this left me with a complete version of the game to gift to a friend, and well as extra figures to gift to my neighbors' kids.
It only gets expensive if you end up in the "I want it now" mode where you need to have all the version of all the figures. While there are marginal upgrades- Wow Pows, heroic challenges, etc...
but that's no different than the extra DLC that you'd pay for in other games (like buying other cars in Forza, etc)
It's pretty easy to be a CAG and still get the bulk of game value out of the Skylanders series on the cheap.