getting subtotal and programmatically changing it

I’ve tried using coupons and different plugins to achieve this but haven’t found any. I’m looking to change the total to pay based on a range of subtotals. For example,
If customer buys between $5 – $275, I want them to pay only $125
For orders between $275 and $650, I want them to pay only $250
etc.
I guess at this point I’m looking to catch the subtotal before checkout and automatically reduce it to the desired amount. Because I want a fixed total, I can’t do it through a coupon, unless I define all the different subtotal combinations and give each one its own form of discount. That would be very time consuming. Can anyone help me or give me an idea to point me in the right direction? thanks