I used kitty litter as ballast on the original portion of my layout when I built it nearly 22 years ago and I've continued to use it on all of my layout expansions.
From a price perspective, I doubt you could find anything cheaper. From an application perspective, as a previous poster has noted, it is dusty but not to the point that I would be any more concerned about that dust than the smoke coming from the stack of a locomotive. If you do opt to use kitty litter, the only cautionary advice I would offer is to make certain that what you buy does not have the little deodorant crystals mixed in with the litter.
Over the years, I've had to remove kitty litter ballast many times to replace track or switches or to tie in newer parts of my layout. All that is necessary is to soak the kitty litter with warm water, wait about 10 minutes, then use a paint scraper to take it up.
I've never had any issue with moisture in the "ballast" nor have I had any issues with our cat confusing my train layout with his litter box. (Now, there was the time he got on top of some train boxes underneath the layout and got himself tangled in the wiring, but that's a story for another thread.
)Using kitty litter as ballast is obviously a personal choice and you do trade scale realism for price but, I have sworn by the stuff for as long as I have been back in the hobby.
Curt
