It appears that zombies don't give any money, they do however give experience and item drops.

I assume this is something to do with the sets.

Yup, there was a bug. The fix is currently in the dev build:
http://ci.bighatchet.com/job/ecoCreature/

Give it a try.

Awesome I didn't know you guys had a CI server (couldn't find it). Will do, thanks.