well, i just ran a test on the map, and the following happened:
for explanation, i'm using a Gigabyte macro mouse, having a macro that hits the left mouse button incredibly repeatedly.
When i sell towers using that macro, the game seems to sell towers 5-10times as often, giving me that ammount of money.
Possible fix would be using the "salvage" option from the Terran Bunkers, and adopt it into the towers. I've never had problem with salvaging.