I've got the PAL DVD-R version of the 8th Anniversary Special Edition. It's the red one on his official site at
http://www.frogpondmedia.com/dbv/index.html , NTSC ones are green. I don't remember what I paid for it, probably more than $20 though.
I ordered it directly from him about 15 years ago but it seems he is not selling individual copies directly anymore: "All retail sales are now handled by online retailers".
I don't know anything about the 21st Century Special Edition, I guess Dave's the best man to ask about that. His email address is on the site.