Does repairing your item using the mending enchantment make it use more xp as you repair it? Also does how many times you repaired it in the anvil affect how much xp is required to repair?
Right now(1.9 Snapshot 15w44b), repairing an item using the mending enchantment, does not increase anvil costs. Mending repairs items at a rate of 2 durability per experience point, regardless of how many times you have repaired it at the anvil.
That, of course, is subject to change, since 1.9 is not officially released yet, but it will probably work this way.