I need to write a program to compute a monthly gas bill

basicily i am having a problem , writing the logic for the program
The gas companys schedule of fees is as follows:

0-80 Cubic metres $10.00 Minium
Next 120 CU metres $0.10 per Cu Meter
next 200 Cu metres $0.05 per CU Meter
400 and above $0.025 per Cu meter


So i am having a problem trying to figure out how to make the program charge According to that schedule of fees/
What decision structure should i use ? can any give me the basic set up ? Thanks