Whenever I repair a tool/weapon more than once the repair price is increased. After 3-4 times, the item becomes "Too Expensive." I had a perfect mining pickaxe (Silk Touch I, Efficiency V, Unbreaking III), and I had to let it break because the repair price went from 9 levels, to 32 levels, and then "Too Expensive." I hope that this is truly just a bug because it takes a serious time to enchant enough books to get those enchantments. I would be fine with the increasing price if it were to cap off at 32 or something like that.