When I was combining bows, the cost switched by 10 levels simply by changing the bows around in the anvils GUI. screen shots in link below.
http://imgur.com/a/6BGk7
MC-9043
Anvil XP cost irregularity
Resolved
Duplicate
0
(Unassigned)
Unconfirmed
anvil, crafting, enchantment, leveling, tools
Minecraft 1.4.7
Retrieved
Duplicate of MC-3880, please use the search function to see if your bug has already been submitted. Currently over 51% of tickets are being closed as duplicate.