Dude, just add a trigger that when a building is finished being built, add income x to total income for player variable y. When a building is destroyed, remove x
x + y
x - y
the building should only add to the original variable. Use player slots or player numbers as the index and make it an array variable. I could make a 1 minute function for you to call up, but it's just too easy, especially if you got one working...
Every time building w/e is finished, add the income it adds to your income variable for that player, all done ^_^