They are all the same... the differences are the actual RO membrane... they are designed and built to a specification.. (to filter out a specific amoutn/size)
Each filter IS tested and rated... they are sorted and placed in 'BINS' if they meet or exceed, they go in bin A, if they just meet, then bin B, and if they are lower then bin C
Some vendors dont care which bin, and some vendors do...
that is the difference in prices for somethings tha tlook identical..
other things that affect eh base cost are the cannisters... some are clear and some arnt.. if they are clear you can see when they are dirty, but that doesnt mean they are bad...??? so it really isnt a good measure or a necessity
Adn teh differeces are minute so it really isnt noticable to us hobbyist.. maybe to coca cola who measures everything, maybe, but to us, nope...
I would suggest
www.filtersdirect.com and look at thier ebay auctions... they will match thier auction price