How to do bulk discounts

[MaritimeSource]

Here's what we want to accomplish:

 

A 10% discount on $500 to $999 orders

A 15% discount on $1,000 to $1,499 orders

A 20% discount on orders with a value at or exceeding $1,500

A 25% discount on orders with a value at or exceeding $2,500

 

Any suggestions on how this could be accomplished? If no contrib is available, could you point me to the code that calculates the sub total and I'll go from there... They only want to see if displayed in the cofirmation screen, which would have:

 

Regular Subtotal:

Discount:

New Subtotal:

 

Thanks for the help!