Have you had it serviced by a Mitsubishi/Ralliart dealer before? If you have then you should have a copy of a previous service sheet and it will detail what they will check and service at every interval. If you don't have a copy then I can email you one.
The price sounds about right but it will vary across the country depending on the labour charges.
If you haven't had your gearbox, transfer box or rear diff oil changed previously then I suggest you get it changed at the 13,500 mile service. Although not required by the service schedule until later on it is worth changing regularily every 13,500 miles on such a highly stressed performance car.