As noted you can trade in your iPad at apple, for credit, or gazelle for cash, though the turn around time is slower with gazelle - they need to ship you a box to send the iPad in, you ship it, then you wait for them to process it. I did this for my iPhone and it went smoothly.
I can places like apple, target, bestbuy all of which offer this type of service giving less money but its instant gratification, especially if you'll be using the funds to buy a new iPad
Apple is offering $185 trade in credit for the old 16GB mini. Amazon (which does not sell iPads itself, only 3rd party sellers) is offering $194 Amazon Gift Card. I took Amazon's offer b/c even though I can't do an actual trade up though them I'm always buying stuff on Amazon so the $ will be spent no prob.
$185-195 for a product that cost $299 a year ago is pretty decent IMHO. Makes cost of "ownership" something like $8 and change per month.