Well. to help out anyone else, about the only budget trackballs I am familiar with are the Kensington Orbits of varying iteration - Opto mechanical (blue ball) or Opto only (grey pearlescent ball- but BOTH Of them remind me of "ALWAYS with Wings" in their contour.) Both can be found for less than $30 even new, I think. Not great, though, but finger manipulated as opposed to thumb. And be prepared to chase the damn ball around the room if you get the opto only or do a hanger mod like I did to hold the ball in place (take a wire hanger and bend it into a circle just smaller than the diameter of the sphere with a wand sticking off of it, then find a non moving part of the plastic to melt or glue the wand to with the circle over the top of the ball. PITA, but remarkably better than being on hands and knees under the table three or four times a week. I dunno why they didn't just finish the damn thing when they made it like the first opto mechanical ones. Budget, I guess.)
You might get lucky if you are a odd hour owl like me and find a used version 5 Expert for 30 ish on the Ebay. (Sometimes I get stuff just by WILLING it with my mind to be listed and generally unnoticed by anyone else.)