help with a formula for working out commission earned...

I'm trying to create a simple spreadsheet for my sales people so that they can work out how much commision they've earned, by inputting the money they've brought in, but I just can't figure out how to do it, despite spending days on it!

For example:

If they bring in anything up to £3500 - then they get 0% commission.
If they bring in £3501 - £6000, they get 7.5% comm
If they bring in £6001 - £8000, they get 16% comm
If they bring in £8001+, they get 25%
etc

E.g. If they brought in £6500 that month, then their commission would be worked out as follows:

(they would get 0% on the first £3500, as above)
£2499 x 7.5% = £187
£500 x 16% = £80
So their total commission for that month would be £267

If they brought in £8000 then it's worked out as follows:
£2499 x 7.5% = £187
£2000 x 16% = £320
= £507 total commission for that month.

How do I create an 'IF' formula in one cell that works out the commission earned, when the commission structure is split into different levels? Is this even possible?!

Any help would be greatly appreciated,

Neil.

or a single formula in a single cell could be something like

=(IF(A1<=3501,0,MIN(A1-3501,2499))*0.075)+(IF(A1<6001,0,MIN(A1-6001,1999))*0.16)+(IF(A1<8001,0,A1-8001)*0.25)

R