No such thing as a "right price"! As bluegrass says, rare to find a Mobo for charter in Corfu, so if somebody is offering a Fairline Targa I guess they can set their own price as there won't be any competition.
To give you an idea tho, a 40 ft sailboat high season would be around £2000 a week, skipper £5-600 per week.
BTW make sure the boat has a proper Greek charter licence and is not just somebody moonlighting.