There's currently somebody on eBay trying to sell a copy of KZK for over $400 USD. I haven't given much attention to the market in awhile, so I don't know how often these vinyl LP's really change hands anymore, or what prices the buyers typically agree-to-pay.
This is not the most thorough search, but should give you some idea of the current shape of the Japanese market. If you get a factory-sealed copy of KZK from there instead of eBay, it will probably still cost you several hundred dollars in the end (after you go through some kind of deputy service). I'm also shocked by what sellers are expecting for Saturday Night Gossip. Used copies of that are even more expensive than used copies of KZK!
KZK has always been cheaper and easier to find than MM or SS, which is probably not what you'd expect if you're an international fan who is new to this market. We'd like to think a KZK LP is the holy grail, therefore it must be the hardest-to-collect Shiina Ringo LP out of all the major ones, but no - not even close. We already knew that the quantities-of-CD's-sold in Japan were much lower for KZK than for MM or SS. What I didn't predict was that the levels-of-demand for the LP's
in Japan would be proportionate with the CD market, but that's exactly what happened, and that's why KZK is the only album among the first three where the supply of LP's significantly exceeds the demand. It's not that the KZK supply was ever any higher than the other albums to begin with - it's that the demand has always been much lower.
If you had to pick just one old vinyl, I would say go for KZK. It has the Casshern (English) version of Kuki instead of the CD (Japanese) version, and they included Ichijiku no Hana after Souretsu. The music is great, the vinyls are heavy, the packaging is arguably the most gorgeous (of the full-album LP's), and this has always been an undervalued vinyl compared to the rest. If you don't have a lot of money to spend but you still want something that looks really nice, then get Mayonaka wa Junketsu. You can see the robot bits in her eyes, without a magnifying glass.
Good luck with your vinyl quest, and be sure to show off whatever you can get your hands on!