I was looking on eBay the other day and there were a pair of K240DFs in reasonable condition, and a pair of K240 Sextetts in mint condition. The DFs went for £25 (not including shipping), and the Sextetts are currently at £37. If you lurk enough a pair of DFs should show up eventually, and you can probably snag them for a very reasonable price. If eBay fails, check the For Sale forums here on Head-Fi, which always offer fair prices.
Keep in mind that the DFs need a LOT of power to sound good, so a powerful (read: expensive) amp is needed. The least you'd probably be spending to get them to sound their best would be ~$270 on a Matrix M-Stage. Out of a headphone out they'll just sound very very average.
Also, welcome to Head-Fi! Sorry about your wallet.