Let's do a quick reference point, Alaska Airlines.

to "F"
0 - 1,250 $50
1,251 - 2,500 $100
2,501 - 3,750 $150
3,751 and above $200

to MCS, "exit row + food + bag"
849 miles or less is $35
850-1499 miles is $55
1500 miles or more is $110

to "F"
849 miles or less is $70 (+35)
850-1499 miles is $110 (+55)
1500 miles or more is $270 (+160)

If you are checking a bag, then the equation makes more sense.

I would like to see <850 $35, <2000 $60 >2000 $85, thought?

always,
m.