Lead is heaviest... should be able to bolt up a container full of lead shot fairly cheap....
Also.. you don't *have* to bolt to the wheels... fix the weight somewhere else... bolt or weld on some nice heavy plate to the underbelly frame for armor and you'll get dual benefit...
A couple of other cheap ideas gleaned from the TBN forums:
Eat a turkey before using the tractor, they're good for at least an extra 10-20 lbs, and there are tons of them walking around (at least they are here). Donuts work well too, but I prefer protein to fat or carbs.
Duct tape your mother-in-law to the ROPS. Father-in-law too, if you need more weight. If using both, it's probably best to ensure they are back-to-back and head-to-toe for balance.