That's correct.
But now i can't do the rest
With:
IIf([CanCost]=True,1,[AntRealCalc])
i got this error:
Syntax error, operand without an operator.
With:
IIf([CanCost]=True,"1",[AntRealCalc])
I got this error: 
"THE EXPRESSION COULD NOT BE SAVED BECAUSE ITS RESULT TYPE, SUCH AS BINARY OR NULL...