No one used Canon Service?
Yes, I occasionally/rarely use Canon service, but not for a decentered L zoom lens.
Why are you asking? Can't they give you a rough idea, or are you not in agreement with their price quote?
I know they need to investigate first, which will cost money anyway, before they can be a bit more specific. They also need to determine the cause for decentering, e.g. dropped on the ground, because that may require additionally replacing bent/cracked parts and further disassembly. And if the lens is in a bad shape, they'll need more than regular CLA effort to be put in.
Cheers,
Bart