well, it'll be an absolute bargain...and more practical...than a Porsche Sport Turismo. But the real competitor is the Mercedes E63 wagon. It'll be interesting to see how the two compare pricewise.
That being said, the RS6 Avant transcends rational and economic logic. It's something all of us have wanted at some point or another, evidenced by the fact that gutting our sedans and transplanting all of the parts into an S6 Avant shell isn't a devaluation or an eccentric decision, but an upgrade.
Cars like this are bought based on emotion alone.