I got a Hyper Zetton on Ebay for about £9 which seems pretty dang sweet to me. I've picked up a handful of the Ultra 500 type figures (Red King EX, Gudon, Baltan and soon Dada) because I like the monsters a lot.
I know practically nothing about any Ultra figures though apart from their basic existence so I don't know what's rare or what's not or what comes from which series, etc.