My patience and savvy trading pays off...
EB had a "trade 3 360 games, get an additional $20 bonus" deal a short time ago. It was just after the announcement of ES4: GOTY edition (which includes The Shivering Isles and Knights of the Nine expansions). So I traded in my old copy of ES4, TR:Legend, and Viva Pinata and put the amount towards reserving GOTY.
So since I never downloaded Shivering Isles or Kot9, I'm getting an additional $45 in content, plus the original game, for $6. Overall with the trade I figure that I've come out ahead about $10 because I knew I wanted to get the expansion packs eventually, now I have them all in 1 place. And if I wanted to rebuy TR:Legend or VP in the future, they're already starting to be marked down.