Well, first of all, congratulations (hypothetically)!

I'm currently contracting thru a small (<20) shop, and our travel policy is our client's policy. Maybe small non-profits are different, of course, but my current client is a large religious organization, and we are traveling within their limits/approvals. So a lot of this may actually become moot.

I would set a fixed limit ($50-100) on airfare differences below which no approval is needed. Even when airlines match airfare, a routing difference may result in a $5 difference due to differing airport fees. It's silly to have to get approval there. Aboce that amount, continue to evaluate on a case by case basis.