I can only sumise that there are more 4 string players than 5 string players. Prices can also fulctuate wildy depending on the time of year, how keen the seller is to shift the bass, originality, if it comes with a case etc. etc.
The one located in Poland is identical to the one I have, I managed to steal mine for a paultry £750.00 via ebay!!! That is no reflection on the value of this bass just an auction with collection only from the middle of nowhere...