I think it's more than that, honestly. The problem is that you usually see auctions for a group of different zombies. I've only seen the elf zombie sell in an auction by himself once, and he went for $23 (incl. shipping). Of course, as always happens with eBay and Mordheim, this is quite likely an outlier not a rule. The zombie dwarf, who I've seen sold 5 times alone, averages $13.50. I would guess that the real answer is somewhere between those two, somewhere between $10 & 20?
It's somewhat rare, but not SUPER rare. It's not one of the ones that elicits a feeding frenzy, like the second generation Sisters, the witch, or the merchant.
Best of luck!