pricey I guess as it's a rather captive market, 'you want these...we have them' type thing, and few if any other retailers do.
I have a 512 deck again feet had perished/sagged so no longer gave any support. I'm going to use some thick dowel (a rolling pin, nice Beech wood!) and add some 'rubber' to the base of these. The original feet were not overly compliant so probably offered limited isolation. Mount the deck on a decent stand and all should be ok.
Other options are spikes, or 'sorbothane' isolation pods, or tennis/squash balls cut in half, hey you wanted cheap!