I like how google did employee travel. They calculated a cap = 2/3 * historical employee data + 1/3 * current cost of business flight.

Obviously for common routes such as Sf to la, nyc to sf...etc they hard coded values.

for every $1 under the cap you received 50cent to use on a future travel plan over the calculated cap.

You could fly economy on an international flight saving money, then bank 50% of savings to buy a first class ticket some other time.

This created a negative feedback loop keeping travel costs in check.