I've just got my 3D printer working quite well. I bought a kit second hand from someone who didn't have time to finish it and after building my own electronics and writing my own firmware for it, I'm probably in to it about $300.
I never really had the intention of printing enough stuff to pay for the printer, but it may still do that. One of the things I'm currently working on printing (printing out a piece a night) is a solder fume extractor. This is a relatively simple tool that sucks solder fumes through a carbon filter and normally costs ~$150 - $300. The one I'm printing should be just as good and will cost maybe $20 including plastic, fan, and filters.
Also, if you want to try and pay off your printer, you could print new printer kits and sell them online or to friends. But, it is ONLY going to "pay" for itself if you don't value your own time, since you will spend a lot of time nitpicking, calibrating, babysitting. Although babysitting isnt as big of a deal, I just try and check on it every half hour to an hour. Many people will even just let stuff print out over night, but I'm not quite at that level of trust just yet. It's not that I care about wasting plastic, but that I don't want to die in a fire while I sleep.
The real advantage to a 3D printer is that you can design and build your own stuff very quickly. If I want to design and build a small robot, I can draw up some wheels, a frame, some servo mounts, etc.. and have a very nice professional looking toy robot in a couple of days. Or if I want to make a simple enclosure for an electronics project I can import a 3D representation of the circuit board, draw a box around it, cut out holes for switches, connectors, LEDs, and print.