What about setting a merc cap,but this merc cap is all about how big your army is..like having an army of 10,would let you buy 1 merc ,army of 20 is 2 and so on.so the number of mercs you can have is all depended on how big your army is. This way having a negative income would be impossible because you can not buy the amount of mercs needed to have a negative income,based on your army size.