No doubt the system has been 'fine tuned' but, in general, before the days of UDU, the carriers would sell 4 upgrade coupons for app $250 to non-elites. Each coupon was good for 500 miles and the agent would take the coupon(s) if you were upgraded. The check-in machines offer roughly the same deal. So a flight of app 500 miles would be app $60-62 - which is close to your $50.