It's being used in the context of someone paying by the ten-minute chunk for a service they're receiving.
The only difference is the place in which they are used:
Clock out: This is used by Americans
Clock off: This is used by the British
On the whole, there is no difference in meaning as both of their origins come from the time clock systems.
They're generally used colloquially, for example:
What time do you clock off? We could go down to the pub later.
I think I might clock out early today...I don't feel very well.