The USB connectors are their own interface board and is expensive. I bought their cheap highlip trackball and ordered a cheap USB interface from Oscar along with a harness (basicly no work on my part).
If you are gonna hack a PC Trackball... see if CompUSA still has their clear trackballs; serial and PS/2 with a 2 1/4" (I think), and only $20. Very easy to hack this one, even top mount is easy as the PCB already has holes for the bolts to go through.
Kensington makes some nice PC trackballs too. I replaced the CompUSA one with a smaller optical USB trackball, $35. The backspin was worse on the optical trackball. Weird. Just took some extra plexiglass attached to the same bolts, to suspend the trackball.
For 2 or 3 times the price though, you may just want to get the Happs 3" while its on sale. If you do a PC trackball hack, you're gonna eventually be dissatisfied and replace it, anyhow.