I think the real thing here is:

0 - 500 $10 unlimited

501 - 1,000 $15

1,001 - 1,050 $20 // $5 = 50 emails

1,051 - 1,150 $25 //$5 = 100 emails

1,151 - 2,500 $30 //$5 = 1350 emails

2,501 - 2,600 $35 // $5 = 100 emails