Well, how cheap? If you can get a broadband adapter for real cheap, then it's probably worth it. Even if you plan on selling it right away, its probably worth it. A dc BBA goes for $100 USD (w/o box, or incomplete) to $150 (complete, or new). I don't see the retail value of these going down anytime soon, either.
And, yes, there are those of us who do still play quake 3 arena on our dreamcasts from time to time!