and it costs three times as much - even though I'm pretty sure it is exactly the same thing
It probably is. My guess is the manufacturer did another clinical trial to obtain 3 year approval. They had tried before but, if I remember correctly, not enough cats died so they didn't get the 3 year approval.
There is no difference between the regular (non-PureVax) 1 year and 3 year vaccine -- they are the same, just approved for 1 year and 3 year "efficacy." (In dogs, they've already found the regular rabies vaccine is effective for at least 5 years. Check out the Rabies Challenge Fund for more info)
I'd be willing to pay the higher cost if it meant I didn't have to do a yearly vaccination, esp. since they're effective longer than the approved length of time. I don't like hitting the immune system that often. Even 3 years is too often, IMO.