Sometimes the economic cost is irrelevant to the ideological purpose.One can only wonder as to how Putin can justify the sort of expenditure given that what he has annexed has been virtually destroyed by war and will cost billions to restore ( if he even get's the chance and has the want to do so)
Either Russia will rebuild that which has been destroyed, if it ends up being part of Russia (Putin has stated as much, I believe, not that his word has much weight these days), or they will have to pay reparations in some form.