I am trying to find upkeep costs for castles, businesses and private homes. I have set annual property taxes at 5% of the total value. I would imagine that upkeep would also be a % of total value. In the 1st edition DMG it states 1%/month. That seems high to me. In The Stronghold Builders Handbook (3rd edition) it has 1-2% annually. I could not find this in the 3.5 DMG. I could have just been unable to find it. Give me some sources that may help me(I don't have every book) or give my your thoughts with reasoning why.
Mmmm not so much info in the 3.5e books. I suggest that you go with Stronghold Builders Guide.
The 1-2% is a guideline for the DM if they determine that the stronghold could not raise enough capitol to keep itself going. Example a desert stronghold without local food or water would incur an upkeep.
However a normal medieval keep with surrounding farmland and a local monster problem charges the local populace to keep them safe or charges them rent. This negates upkeep costs.
Unless you are making crazy demands about what is in your stronghold I would suggest that there is no upkeep for the stronghold itself; staff is another kettle of fish entirely.
You do have to worry about staff costs. Per month 3gp for maid and 6gp for a guard adds up real fast for a medium sized keep.