Most DVD's aren't worth crap. Best bet is probably going through Amazon trade and finding the ones that are actually worth anything (as someone noted these prices change). The rest of it you can just sell in bulk lots, but be prepared to likely not even get $1.00 per disc for most DVD's. You're other alternative is to trade some of the worst ones on DVD2Blu.com, at least then you can use the DVD's as trade to get $4.95 blu-rays.
Anyway even though Amazon prices are mostly not even worth the effort to put them in a box (i mean seriously, does anyone trade at $.25 per disc?) I was at least able to find a few "run of the mill" DVD's that were trading in the $2-$5 range, even ones that would most likely not fetch that on ebay or anywhere else for that matter. There doesn't seem to be any logic to it, but I'll bet if you go through your entire collection you will at least find a handful of surprises with the value of amazon trades. Just keep in mind you'll probably go through 10 movies that are worth $.25 before you find one random one that is for some reason $2, or $4 or whatever (Bronx Tale for me was $5.20, Spartacus $4.75, Lawrence of Arabia $4.50 - those are the highest ones I can think of as far as just for a single movie DVD). One thing these movies have in common is they aren't available on blu-ray (yet), so that was probably a good time for me to get rid of them. I don't know if going through your collection 1 by 1 is worth your time, but I was more than happy to cash in about $100 via the Amazon route.
Obviously Criterion's and box sets you should expect to get more. Sadly even those are mostly down in the sub $5.00 range. I don't feel it's even worth it to trade a Criterion for under $5.00 unless it's something I've already double dipped with a blu-ray purchase on...