Building Costs:
Harbors (Free in existing cities)
Small farm: 300 to build, 100 to maintain
Farm: 500 to build, 200 to maintain
Large farm: 700 to build, 300 to maintain
Small fishery: 300 to build, 100 to maintain
Fishery: 500 to build, 200 to maintain
Large fishery: 700 to build, 300 to maintain
Mine: 400 to build, 100 to maintain (provides iron, access to which is required for building heavy units, but not for maintaining them)
Gold Mine: 400 to build, provides 100 coin per turn. Must know of a gold deposit.
Workshop: 500(?) to build, provides 200 income (requires iron to function)
Mage Academy: 700 to build, 400 to maintain. Allows for the creation of mage units.
Vineyard: 200 to build, 100 to maintain. Allows for wine production, a valuable trade good. Must have proper climate.
Quarry: 400 to build, 100 to maintain. Provides marble, a trade good and above average building material. Must have marble in your country.
Units:
Militia: 100 to build, 100 to maintain.
Soldier (and variants): 300 to build, 200 to maintain.
Mercenary: 500 to build, 300 to maintain.
Slayers (and variants): 500 to build, 300 to maintain.
Slave units: -100 maintenance cost
Ships (and variants): 300 to build, 100 to maintain.
Scholar (and variants): 500 to build, 100 to maintain
Mages (and variants): 600 to build, 300 to maintain.
Ok, this is the official ruling on costs. I think I may have told people varying costs for things, but this is what it is. Special units may have slightly varying costs.