Here is a list of materials and costs: 
* Gooseneck coupler ($100, ebay), 
* two feet of 3" square tubing ($17, local steel supply), 
* four feet of angle steel ($10, local steel supply), 
* 2" ball ($11, ebay), 
* Three Grade 8 bolts ($6, Tractor Supply). 
I've got a guy that works for me around here that did most of the fabrication; I probably paid him $100 for his labor. All in all, I've probably got about $250 in it. If you have the equipment and the skills to do it yourself you could have saved the $100 labor. Also, if you didn't need the hitch to be adjustable, you could just buy the gooseneck coupler/latch and fabricate the rest (and probably save half of the $100 I paid for the whole coupler assembly). 
I chose to remove the coupler latch pin that came with the coupler because it used up about 6" of the possible height adjustment. I replaced it with a simple pin and key from Tractor Supply.