The current price is probably just an artifact of preorder prices
Sinkhole can be a great tempo card to put your opponent a turn behind, or ideally, a couple turns behind if they were already struggling with their land drops
Hell Mongrel gets the spot on the list over cards like Terminal Agony and Bone Shards because it does both sides of what the deck wants
Suddenly, all that discard, land destruction, and card drawing they've used has backfired, as they find themselves at a low life due to Necropotence, with two (or less) lands in play, no creatures, and no cards in hand
There's a reason we say everything dies to Doom Blade. #11
Its also cheap enough to have in hand to bump your Commander into Commander Damage range a turn sooner than an opponent suspects