Look in Edi's Database
http://forum.shrapnelgames.com/showthread.php?t=42819
Find the unit you want to know about. Look at column 'AE', which shows a units gold cost to hire.
If the figure in this column is zero, then there are no upkeep costs for that unit. If there is a figure in this column, then divide this figure by 15 for the monthly upkeep costs (or divide by 30 if the unit is sacred)
From Edi's Database, Maenads have zero cost to hire, so have zero upkeep costs. Mictlan Slaves have a hire cost of 2, so upkeep is just above 0.1.
Edit: Triple Ninjad. Ahhhh FFS!