Please Help!!!

I am terrible when it comes to creating forumlas in Excell. My company is switching to a different, tiered based commission for our sales people.

We are a cell phone company, and our commission will be a sliding scale. The more gross profit the sales rep brings, the more they get paid for each phone sold.

Here are the ranges:

$0-500 GP $8 per phone
$501-$1000 $10 per phone
$1001-1500 $12 per phone
$1501-2000 $15 per phone
$2001-2500 $18 per phone
$2501+ $20 per phone

example:

John brings in $1700gp, and sells 17 phones, he would earn $255.
John brings in $900 gp, and sells 5 phones, he would earn $50.

We have 70 sales reps.

I really don't expect anyone to do the work for me, but if anyone can at least advise me where to start, I would really appreciate it.