Your best option depends on whether you are buying for PS3, Wii, or Xbox 360.
Once you get Spyro's Adventure you DO have a portal, *but* if you're buying for Wii or PS3 it is a battery operated one which means you will end up spending additional money.
The second game (Skylanders Giants) is $60 for the game + a copy of TreeRex, or it is $75 for Game + Portal + TreeRex + Jet-Vac + Cynder.
The characters are $10 a piece, so if you are getting the PS3 or Wii version and only have the battery operated portal, you're saving $5 off an additional character and lord knows how much on batteries.
If you're on 360 it's a tougher call and totally up to you. Gopd luck!