I have a calculation that has the following charges (2 sets) to be applied
that vary depending on the quantity. What function can I use to calculate the
total easily and apply with whatever quantity is required? (please note, the
charges apply in a incremental way ie. in set 1, for a quantity of 23 the 1st
is charged £645 then the next 9 are charged £495 then the next 9 are charged
£470 then the next 2 are charged £445 which = 23 in total and a total charge
of £20440, then set 2 would need to be calculated as well)

Quantity = 23

Set 1

Quantity Charge
1 £645
2 - 10 £495
11 - 20 £470
21 - 30 £445

Set 2

Quantity Charge
1 £350
2 - 10 £250
11 - 20 £237
21 - 30 £225

Any help would be greatly received as I can't think of how to approach this.