Excel sheet where a user can enter the number of electricity units used, and display the total cost

This is the question:

If 500 or fewer units are used, the cost is $0.02 per unit.  If 501 – 1000 units are used, the cost is $0.02 for the first 500 and $0.04 for the remaining.  If more than 1000 are used, the cost is $0.02 for the first 500, $0.04 for the next 500, and $.08 for the remaining.

a) Create a spreadsheet where a user can enter the number of electricity units used, and display the total cost.
This is my formula so far:

B8 is the units
B3 is 0.02

B4 is 0.04

I'm stuck on the last line of the question. Can I get some help, please? Thank you!