Perhaps the OT would have a source for getting the pellets at a wholesale price?
Consider the fillings ... beans explained above.
Rice (and any other food stuffs) ... might be of interest to mice.
Sand ... hhmmmmm? wonder if the cat would figure it out? and if it should get wet, and soggy?
Despite the costs, I think I would opt for the artificial ... besides, I'm sure you'd agree, that for your child is well worth it.