I guess this falls back onto what 1 'unit' is. I a unit of
lumber is a tree (seems logical) then a unit of stone could be
related to a 3' cube (about right for castles/pyramids etc ?),
gold is easilly just gold (but what about minting it), and
horses is horses.
I think you may want to include ore in here as well...
What sort of output in gold should be possible for a task in a month?
5,000? 10,000? Right now gold is about 30,000, which seems quite high,
even to those earning it.
It should be related to the costs in learning the skill ($) and
time taken to learn. Say working for a full month should be
enough to support your self in training for 2?