If you're buying new Kubota weights all you can do is check several dealers and try for a better price.
If you either make or have someone else up the bracket for you, it's possible to find all sorts of weights in tractor scrap yards. The other idea is to rig up a bracket that will take used weight lifting weights. There's any number of ways to add weight to the front. You could rig up a concrete block with embedded brackets and detachable legs that you drive up to and pin on.
An expensive way would be to foam fill the front tires. That would probably add 100 to 200 lbs. If you have a loader
the loader frame is going to complicate things.