I currently have 48 xp and I am attempting to repair my diamond pickaxe of Efficiency 4, Unbreaking 3, Fortune 3, and the anvil says it's too expensive. I have never used this pickaxe on the anvil before... Is this a bug?
Did you enchant it before the enchantment levels dropped to 30? If so, it's impossible to get enchantments like that in the game now, because enchantment tables don't give them and the anvil caps the enchantment levels to 30.
The level cost on an anvil is capped at 40. Any combination or repair that costs more than 40 levels will never be possible in survival mode, no matter how many levels you actually have. There is no cap in creative mode.
Each time you repair or combine an item, the level cost increases by 2. You can mitigate this, somewhat, by renaming the item first. This will add some to the level cost for future repairs/combinations, but it makes the item exempt from the increasing cost each time you do.
You used the anvil wrong. The first thing you should do with a great diamond tool is rename it. This mitigates the cost and means that as long as you don't drop it in lava or the void, you can theoretically keep repairing it forever. I have a full complement of renamed tools with great enchantments in my single player world, all renamed as soon as I got the enchants I wanted, and I don't anticipate losing them any time soon.