Sounds to cheap but if its just purely the cambelt kit and not the accessory belt and other serviceable items then that will be why.
I priced up a full 5 year service today for my car and that came to over £400 for just the parts, that was with the cambelt kit, accessory belt kit, water pump, oil filter, washer, cabin filter, coolant so add another £40 for oil and then labour from a main dealer at £60 odd an hour you are probably looking closer to that £600 mark.