It'd probably be one of the easier sets to complete. There's only 247 US titles and nothing I can think of that'll ebay for over $100. Once you get Powerstone 2, Project Justice, Skies of Arcadia, and a few others, you'll be on to collecting cheap games.
I'm not a fighter fan so the dreamcast collection has limited upside for me personally. It does feature some great shooters and two of my most favorite rpgs (grandia 2 and skies of arcadia) but there's a whole lot of outdated sports games (the 2k series was great but is now nearing 2 decades old) and 3rd party games (a la tony hawk) that haven't stood the test of time, not to mention some light gun titles that need crt tvs to play.
With all that said, if I was attempting a complete collection the Dreamcast would be it. If you're limiting it to a US library and was patient (and a little lucky) I'd bet you could do it under $5,000. If everything had to be complete and perfect I'd bet you'll still come in under $10,000. Even if you average paying 25 per game that's around 6,800 bucks. If it's still possible to find things out in the wild you'll do a whole lot better. I've purchased 50 dreamcast games + games, controllers, etc. for under $100 at a yardsale before. One lucky grab like that is 20% of your collection.