I've got the GME PLB with GPS, was around $600. I'm told it meets aviation requirements for mandatory carriage, and in any case next outback flight I'll carry the big one out of the boat as well.
Two important points.
Please, please disable your old EPIRB before getting rid of it! By "disable" I mean kill it, cut it in half, remove the battery and do your best to destroy the circuit board. Idiots will still play with these things if given the chance.
Secondly, register your new EPIRB on the AMSA website, even if it's a small PLB. The website allows you to add multiple primary and secondary uses for the EPIRB such as aircraft, boat and automobile. They will send you a nice little sticker to apply to your PLB to prove it's registered.
If it's not registered, your chances of a prompt and appropriate response if it's triggered are not that good.