Economies of scale, size of the ball, and high durablity ("arcade quality") demands, to name a few.
Economies of scale:
The more made, the cheaper per device it is to make. Thus the ?200? a year arcade trackballs cost more per than if ?10,000? were made, on the generic mice scale.
Example why: Even before a single trackball or mouse device it made, molds must be consturcted. Molds cost thounds of bucks. $2000 divided by 200 trackballs = $10 per TB just to cover the molds (assuming two molds @ $100 bucks each, which is pretty conservative). $2000 / 10000 mice = $0.20 per mouse to cover the molds. (molds do last over a few years, but this is just an example)
Size of ball:
High quality, near perfectly round balls are hard to make. The bigger the ball, the more it costs. Compare a 3" TB ball to a 1/2" ball used on a mouse. The cost is not linear.
Arcade quality:
Arcades quality equipment need to be able to survive a lot of pounding. PC devices don't. You pay for the increase in quality.
And high quality PC trackballs can cost more than arcade TBs. Kensington Expect Pro & Turbo Pro Trackballs (2 1/4") cost ~$100. You can get ps/2 3" betson trackballs for ~$80, arcade wired wico 3" costs ~$50, and wico/suzo 2 1/4" trackballs cost ~$25. Shop around.