Banding Numbers Associated Trigger Points?

We have a customer rebate in place with various levels of refund based on the quantity purchased during the year. I have used a sumproduct formula to calculate this before.

The customer used to have the following set up -

0-999 - £1.00 per unit rebate.
1000-1999 - £2.00 per unit rebate.
2000-2999 - £3.00 per unit rebate.

So if they bought 2501 units they would get a rebate of (1000*1)+(1000*2)+(501*3). However the customer has trigger points so rather than the above it is now -

0-999 - £1.00 per unit rebate.
1000-1999 - £2.00 per unit rebate.
2000-2999 - £4.00 per unit rebate if 2500 bought.

So now it would look like this - (1000*1)+(1000*2)+(501*4). However if they only bought 2499 units it would be (1000*1)+(1000*2)+(499*2).