Ideally, prices should be consistent with effective costs. Practically, I would be content if prices of any single leg couldn't be negative. If leg A costs a a

Ideally, prices should be consistent with effective costs. Practically, I would be content if prices of any single leg couldn't be negative. If leg A costs a and leg B costs b, the cost of A+B can be lower than a+b (you can offer bundles, discounts, etc.); just not less than either a or b. If you play "tricks", the consumer has the right to use such tricks to their benefit.