Hi Vic, I couldn't really comment on the collectable nature of these. I suppose one factor would be if they change from edition to edition. I.e if the 12th edition contains everything up to and including the 11th edition plus anything new since them if I were in the market for one I would just buy the most up to date. But each to their own. You are right though some of the prices on Amazon seem crazy, but you can get a 'good' used copy of the 9th for £41.

https://www.amazon.co.uk/gp/aw/ol/19...&condition=all