Is there a better way to buy Adobe Illustrator?

I’m a little low on cash, but I still need to get Adobe Illustrator. I plan to buy Illustrator CS on ebay for $180 and then upgrade it to CS4 for $199. Is there a better/cheaper way to do this?

Somewhere along the road I plan to do the same thing with Photoshop.

EDIT: I’m on a Mac, if that helps.