Just ignore it - it's a vanilla thing that Aikar's made not matter. It'll probably cost around 40+ levels to combine though.
I just went in a SP world and tryed how hight it can go, I reached up to level 86. It has full entchanting and got another name
as far as I see on my tests they dropped the normal amound of items ( skeleton 1-2 bones/arrows; sheeps 1-2 wool; ...) [edit]: some drop nothing
you can add it with the entchanting books or with TooManyItems. with TMI you can entchant up to level X, loothing X: one skeleton dropps 9 bones and 3 arrows. another one 4 bones and 6 arrows
ANVILS: In creative, anvil creations will never be "Too Expensive". however, in Survival, there is a level cap.
Also the order of the bows enchantments matter. I remember combining two picks. They were like Fortune3, Unb3 Eff4 and the other Eff4 Unb3 Fort3 depending which went first the level affected it highly. I had a difference of 6 levels if I well remember.