I would guess this is designed to feed one or two 12v batteries to power the usual camper-conversion amenities, which would be the main market for the extra alternator. A fridge, a couple of lights, maybe A/C and a microwave and all of a sudden 90 amps doesn't sound so overkill anymore.
About the price, Zurich (where Gut is based) is regularly ranked the most expensive place in the world for the cost of living. They do some very good quality work but they pay local-rate rents and wages, same as their suppliers, no surprise it ends up being passed on to the customer. As an example, I ordered a side panel for my 710m last week, the original one is rusted and repairing / repainting is not the cost-conscious way to do it. Gut sold the panel to me for about CHF 420 (these days Swiss Franc is on par with the dollar) while SAV has them for US$ 170... Shipping costs tilted the balance back in Gut's way, but not by much... plus the fact that shipping large parcels between continents is not exactly good for my carbon footprint (which is big enough already with my carbureted toys...)
