How to make interactive formula

Hi, I am trying to make an interactive formula and it would be great if someone could advise me on doing this.
table.jpg
This is a piece of the order form I'm making in Excel 2007.
So, I have the 'MSRP' which is the retail cost of the product, then (3+) which is the cost of the product if the person buys 3 units or more (this is 50% off MSRP), COST (2) if someone buys 2 units (this is 30% off MSRP) and COST (1) if someone buys just 1 unit (this is 15% off MSRP). Another field is TESTER which is supposed to be 60% off MSRP.

What I want to do is have the person enter the quantity they want to buy into the QUANTITY field and have a formula which calculates the total cost of the purchase (TOTAL field). So if they enter 1 into the quanitity field, I want it to show 1x $8.50 (COST (1)), if they buy two, I want it to show 2x $7 (COST (2)) and if they buy 3 or more I want it to be 3 (or whatever number) x $5.00.

Can someone please advice if there's a way to do this? Thank you very much!

Something like this?
Place in total column, change cell references to suit.
=IF(E2>=3,C2*E2,IF(E2=2,B2*E2,A2*E2))

Windy