CDRLabs.com

This one was sold by someone for more than $2,600!
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4315&item=8136008689&rd=1
The unit and the box aren't even in excellent condition.

I checked mine (with original box and instructions) and it appears to be worth between $60 and $80 (seen ending prices from $31 to $110 depending on condition of box and country of sale etc.).
I'll keep it for nostalgia sake.
EDIT: It appears the presence of a box and its condition make a big difference in the price; I knew they were important, but it seem to make the toys worth a heck of a lot more (I thought about that some more while noticing 6 Million Dollar Man dolls up for auction.
I believe ones with a box in excellent condition can fetch 5 times the price or more.

My two brothers and I (young at the time, so we didn't think of putting the games in a suitcase) had some Game & Watch toys stolen by a hotel employee while we are at the pool or eating a meal.
Those (I forgot which games) might have been worth more now than DK.
I think Flagman was one of them!
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4315&item=8136146751&rd=1

Soemone TRIED TO BUY the empty carton, that simply holds ten games in their retail boxes, for $150, but the reserve wasn't met!

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=2541&item=5938609544&rd=1

Speaking of boxes, not cartons:
I usually keep boxes, but that is more to protect items during moves; I never knew, till pretty recently, that the box could be the most important part in determining the resale value.