Pricing with flat gp cost abilities?

How do I calculate the cost of upgrading a piece of equipment with a flat gp cost ability? It costs the same amount to add, for example, the Expeditious ability to a set of +1 armor as it would to add it to a set of +3 armor. Expeditious, disregarding its price, counts as a +2 equivalent ability.

This, to me, is fine for factoring into the maximum effective armor bonus a piece of equipment can have. However, if the Expeditious ability were to factor into the price for further upgrades to the armor, such as upgrading the +1 Expeditious armor to +3 Expeditious armor, it would create a price discrepancy of over 40% compared to simply forging a new set of armor, enhancing it to +3, then adding Expeditious to it, and that price gap only gets wider as the enhancement bonuses get higher.

Is this price gap intentional, or do flat cost abilities not affect the price of upgrades?