I used to think along the same lines. Surely such a huge difference could not be justified. After having been in limbo for four years ( before I was rather addicted to the PA50/PA80) I started out with the PA500. Within six months I sold it and got myself the German version instead, the PA500musikant. Why ? Not because I am enamoured with German schlagers etc.etc. but for the simple reason that the Musikant version held approx. 50 extra multisamples, 20 organ orientated, 10 drums samples (much better snare drums than on the standard PA500) and 20 various, inlcuding better trumpets, a meritable Les Paul etc.
The amplification of the PA500 is the first area in which the PA500 is considerably less than its PA800 counterpart, which I now have owned for well over six months. Second there is the keybed and aftertouch.
Third there is the mike in with vocaliser. Fourth there is the external sampling and Ram 64Mb memory. Fifth and perhaps most important of all the total number of multisamples is more extensive than on the PA500 and (though I have no numbers here to substantiate my claim) I think that probably the internal (multi) sample memory is probably twice the size of that on the PA500. Subsequently the PA500 has to sound less than the PA800, though it holds its own quite well and is still a steal given its price tag. Here in Europe the PA500 is about half the price of a new PA800. Whether that is worth the difference is a personal matter. I for one am extremely happy owing the PA800.

regards,
John