Need help with sliding scale compensation formula

Need to create an excel formula to calculate bonus based on a percent of revenue. Here is what I am looking to do in "pseudo-code":

Revenue
$0 - $500,000 - bonus = x% of revenue
$500,000 - $1,000,000 - bonus = y% of revenue
$1,000,000 - $1,500,000 - bonus = z% of revenue
and so on....

The tricky part is that while this is a tiered scale, it still needs to continually calculate revenue to date. Example, the company does $1,250,000 in revenue. The calculation would be something like:

The first $500K in revenue is bonused at x%, the second $500K is bonused at y% and the last $250K is bonused at z percent.

Any help will be greatly appreciated.