Interval pricing of volumes using IF formula

Hi Peeps,

I'm trying to create a simple IF formula to price monthly volumes at the correct rate depending on the volume interval it falls into. I've attached an example, intervals below.

Rate Interval
£3.00 0 to 80,000 Tonnes
£4.00 80,000 to 120,000 Tonnes
£5.00 120,001 to 240,000 Tonnes
£6.00 240,001 to 400,000 Tonnes

as per the example, if in january i sell 15,000 units i would recognise sales of £45,000 (15,000 x £3).

What makes it tricky is that the unit sales are cummulative basis so if say by during the month of June i exceeed 80,000 unit sales for the year a portion of the June sales will be priced at £3 and the amout above 80,000 will be priced at £4 and so on.

Hope this makes sense.

Cheers
Neill