Surprisingly enough, the "Bob" punching dummy seems to stand up to a good beating too. But for those who can't afford to replace bags as often, or don't want to worry about damaging their baton, I suggest some ratan sticks of the appropriate length of your baton to practice with. They can be rough on the bags too, but not as bad as the batons usually and they're super cheap to get and/or replace.
For those of you who don't want to buy/hit a punching bag at all for stickwork, a 4x4 post with two bags of concrete holding it up and old tires placed over it. Really, no joke, it's wonderful, and if acquired right, you only pay for the concrete. If you manage to wear those tires thin i'd be surprised, and tire shops will give you tires they would have disposed of anyways for free.