Cheapo Air: $49.95 +.25M=C
Discount Airways: $89.95 +.2M=C
Let's test them both.
49.95+.25*730 = 232.45 is the price with Cheapo Air.
89.95+.2*730 = 235.95 with Discount Airways.
The equation for the cheaper one is Cheapo Air: $49.95 +.25M=C
You save $3.50 going with Cheapo Air.