Increase low-weight monster Gozag goldification

Switch the baseline mass from 200 (rat) to 550 (human); change
baseline gold from 3 to 13; change weight scale factor from 27 to
40. (Sorry!)

All monsters human weight or smaller now give 6-19 gold (was 1-4
for the smallest monsters); monsters larger than humans now give
somewhat less. (E.g., yaks are down from 18-55 to 16-48.)

This is probably a Gozag buff, but is mainly intended to make
Spider a little more fun/rewarding for Gozagites.