One just opened near me.
There's your problem, right there. The new stores don't have old stock of traded-in Dreamcasts to put in their 30% off bin. We just had another Gamestop open near me, and they hardly have any DC stuff, and the stuff they do have is new/full price. Plus, this sale has been going on for quite some time, at theast the $19.99 part. The 30% off is only about 2 weeks old.
Stores in the mall are far less likely to have any DC stuff, as they get a lot of traffic, and thus turn their inventory faster. Older stores in strip malls are a little more likely to have them. I grabbed the last one at the older GameStop that I went to, and he said they wouldn't be taking in any more.
You may also try GameStops that used to be Funcolands. It's a bit of a long shot, but while the company was still split into different stores (Babbage's, Funcoland, and Gamestop) Funcoland's emphasis was on older systems. People may have been more likely to trade in their DCs there. Like I said, it's a long shot.
I will be scooping up any DCs that I find around here. If I have any luck, I'll offer them up to folks here for what I paid + shipping.