The anvil allows the creation of highly enchanted tools and weapons by use of enchanted books. I created a sword with the enchantments Sharpness IV, Knockback II, Fire Aspect II, Looting III; Everything except Fire Aspect II was a lucky enchantment, and Fire Aspect came from an enchanted book (at a level cost of only around 20 levels). However, as the sword is close to being completely broken, I would like to repair it. Here's the problem:
-> Repairing with the tool material (diamonds) is "too expensive"
-> Repairing with an unused diamond sword is "too expensive"
-> Repairing with a heavily damaged diamond sword (damage value 1560) is "too expensive"
Hence, it is completely and utterly impossible to repair this sword in survival mode.

I went to creative mode to check what the repair costs would be:
-> Diamond: Up to 50 levels
-> Unused sword: 47 levels
-> Heavily damaged sword: 41 levels

What annoys me is that the game decides for me when repairing a tool is too expensive. 50 levels is a price I am willing to pay for repairing my tools and armor, and I find there should be absolutely no reason for it to be impossible to repair tools of any kind, even if the cost for doing so approaches 60 or 70 experience levels; especially if the tool was obtained completely legitimately in survival mode.

My suggestion: Remove the level cap and increase repair costs ever so slightly.