solver : changing value for costs.

How do you add it in a Solver model when the cost of a variable changes within ranges.
for exemple if x <100 then the cost per unit =50, in the range 100-200 the cost is 40 and when x>200 then the unit cost is 30.
Solver 'll think there is no linearity and aborts.
Do you make 3 binary variables, 1 for each range ?