Since the snapshot 14w04b the xp cost increases excessively after repair an item. This not happens in the snapshots 14w02c and 14w03b, and they did not mention any other changes in the anvil mechanics in the change log of 14w04b so this must be a bug. Please see the screenshots where i repair a iron sword (is not even enchanted and happens with enchanted items too) the xp cost are lower in the first, secon and third repair, but it seems to increase exponentially:

first time: 2xp
second time: 3xp
third time: 5xp
fouth time: 9xp
fifth time: 17xp
sixth time: 33 xp
seventh time: too expensive

As we can see this is excessive and the sword is not even enchanted, in the wiki mentions that in 1.8 tools can be repaired longer and with better enchantments, so this is definitely a bug. In the first snapshots 14w02c and 14w03b the penalty increases just +1 or +2 when i repair a tool , so the problem start in the snapshot 14w04b. Sorry if i wirte something hard to understand i do not speak english.