I had a blue "brick" DS and sold it on amazon for about $110.
Periodically, Circuit City has a $40 off of $200 coupon. I used that and picked up a DS Lite and some miscellaneous 360 games.
The next day, I returned the 360 games (They were "gifts" that I found out the recipient already owned...wink wink, etc.).
Total spent, say minus $5 for gas?
-$15
Honestly though, I prefer the old DS. I wish I'd held onto my brick for a number of reasons. It’s sturdy, for one. The system can take a beating; while I take care of everything I own, shit happens and I like knowing it isn’t going to ruin my day if it gets dinged or dropped.
The Lite also smudges and shows fingerprints like mad, and even things like pass-through cases don't eliminate this. I have a couple of really nice cases and some adhesive protectors (Nintendo makes some clear ones that adhere to the screens as well as the top of the system) on the DS itself, and while that helps, it’s an extra $10 that I didn’t need to spend.
I hate the GBA slot. Hate. It makes carrying Advance games with you impractical and (to me at least) limits the totality of the DS’s functionality.
Finally (and this is just personal preference), I just don’t think it’s as comfortable. I don’t have big hands by any stretch of the imagination, but playing on the system without a pass-through case (which adds bulk) makes my hands cramp something fierce.
All that for a brighter screen just doesn’t seem worth it to me; not in terms of cost (hell, I made money off of all this) but in my enjoyment of the system.