The difference between the aqueous and organic electrolyte versions is only the O-Ring material (FKM/EPDM and FFKM, respectively) and the reference electrode type (Ag/AgCl and Ag/Ag+, respectively). FFKM O-Rings are much more expensive. That is why the price varies. Please note that the organic electrolyte version is also suitable for aqueous electrolytes. In that case, you would need an additional aqueous reference electrode.