Parelli tools are expensive, but they last forever. I have had my same halters/ropes for over 10 years, they are still in great shape, but have been well used. I have also used knock-off tools, but they are not the same.
As for training material, IMO the best parelli investment is the original western horseman natural horsemanship book... available online for $10-20. If you really want to go big, try to get your hands on the original L1 Partnership package. Pretty much everything else is high priced and prettily packaged, but filled with mostly fluff, not substance.