There is no course to become a CPL instructor, you just have to be accepted by an FTO providing CPL courses. You don't even need to remove the instrument training restriction.
Don't know what they earn, but would guess £30/H upward.
However, don't get exited too quickly. Unless you have a fair amount of instructional experience most schools won;t look at you because you are too much of a risk to their statistics and pass rates.
Cheers
IRRenewal