Numbers formula needed please!

I need a formula that has a graduated rate of percentage added???

for instance if 1-300 orders are made, then 10% bonus
if 301-600 orders, then 12% bonus
if 601-900 orders, then 14% bonus...and so on.

then i need them all added together.

i do not want the original 300 to have higher than a 10% bonus.

so example

300 orders at 10 dollars x 10% = $300
orders 301-450 at 10 dollars x 12 = $180

total bonus = $480.

please help...