While playing vanilla survival on an SMP server, I noticed while enchanting that the enchanting costs correspond to the slot it's in regardless of the level cost it says in the lower right-hand corner
MC-110993
Enchanting appears to be broken
Resolved
Invalid
0
The 7/13/30 levels is the minimum number of levels in order to select that enchantment. The 1/2/3 levels is the cost of selecting that enchantment.
For example, if you had 35 levels, you could select the final enchantment and you would be left with 32 levels.