You can charge a lot more than that for colour, but I agree you can't have a set price for colour remover as a stand alone treatment because it's not a stand alone treatment.I think colour removal is a precarious thing to charge for, because they aren't going to leave without their hair being corrected after the colour has been removed.
So its a complete service and not just a single service. A full head tint is a single service.
This service charges £25-45 area depending.