In order to mask the Base Price I was trying to come up with a formula that will only show a zero output when the customer inputs a Final Price of zero.I think there is some confusion concerning when the output is 0. As you wrote in the original post, when the base price is $5000, the output $0 happens when the input (final price) is also $5000. When the input is 0, the output is -2500.IF((Final Price- Base Price)*50%)>400, (Final Price- Base Price)*50%, if not then the output will slowly taper down to zero and only be zero when the input is zero.
Concerning "slowly tapering down to zero", the expression (Final Price- Base Price)*50% tapers down from 400 to 0 when the Final Price changes from 5800 to 5000 (again assuming the Base Price is 5000). So, when the Final Price >= 5800, you want to use this formula, but what behavior do you want when the Final Price < 5800?
It may also make sense to ask this question in the "Business Math" section of this site.