Unarmed ships had zero upkeep in DL and DA, but have the same relative cost (as % of initial cost) as combatants in TA.
Unless of course this has been changed since beta 4.
That got changed back to 0 upkeep since then.
Beta 4b:
+ TWEAK: Ship Maintenance now excludes freighters and is lowered by 1%.
Beta 5:
+ TWEAK: Only constructors, terror stars and attack ships get charged maintenance now
So I was wrong in my non-combat ship statement, constructors get tossed into that pile as well for paying maintenance still.