I want to shink a % as the value grows

Question

I am trying to understand a maths problem to figureout an automated fee calculator.

 

I want to base a fee on a % of cost, however as the costs increase i want the % figure to decrease.

Answer 1

f represents fee

c represents cost

Assume

f = 0.05c

For every x increase in cost you want the fee to decrease 0.0005

f = 0.05 - 0.0005x)(c + x)

 

Good?