In season 1, Episode 6 of Lost, John Locke tells Charlie that the Island may give you what you want, but you have to give something up to the Island.
Locke: Do you want your guitar?
Charlie: [nods head]
Locke: More than your drug?
Charlie: More than you know.
Locke: What I know is that this island just might give you what you’re looking for, but you have to give the island something.
Charlie: [hands over drugs] You really think you can find my guitar?
Locke: Look up, Charlie.
Charlie: You’re not going to ask me to pray or something?
Locke: I want you to look up.
It is previously revealed that Locke had been paralyzed from the waist down prior to coming to the Island. Upon arrival on the island he is shown to have regained the use of his legs.
My question is; What did Locke give up to the Island in exchange for the use of his legs?