You are using:
24:3.25,50:4.25,75:5.25,100:6.50,1000:7.75,1500:9.00,2000:12.00,2600:15.00
$0.00 - $24.00 = 3.25
$24.01 - $50.00 = 4.25
$50.01 - $75.00 = 5.25
$75.01 - $100.00 = 6.50
etc etc. etc.

What results are you getting?