Always worth asking the main dealer for a price on a genuine, then search out main dealers who offer discounts for online sales , some are quite reasonably priced (some Merc's barely more expensive than a pattern) though whether even a genuine part will last quite as long as the original system is questionable.

My experience of Japanese OE systems is that they last really well, usually around 10/12 years even if left untreated, but the cars last longer, and an OE exhaust can cost more than the car itself to buy, case in point there wouldn't have been much if any change from a £bag 'o sand for an OE system on our Outback, almost impossible to find the correct pattern part due to the rear silencer on the H6 having a by pass section which opens under pressure, hence why it now has a full cat back stainless system supplied and fitted for less than £300 with lifetime guarantee, when the Landcruiser system eventually breaks up that will be getting the same.

No mild steel standard replacement part is going to last like an OE in my experience, though if you paint the thing properly before fitting it stands a better chance.