The summonable bosses need materials and a few minutes. The ubers need a build that can survive the fight, which is a different thing entirely and priced accordingly. Both are on this page because a buyer thinks of them as the same shopping trip, but the card tells you which one you are looking at.
Duriel, Andariel and the ubers after them are the only reliable source of mythic uniques in the game. If you are here for a specific item rather than a kill, the mythics page sells them directly, and for most single items that works out cheaper than farming the attempts.
Nothing is traded off, vendored or salvaged. A boss run that funds itself by selling your drops is not a service you are buying, it is one you are paying for twice, and we do not run them that way.