Around the set's release, they were fairly easy to buy, but since the hype train left the station, there's been nothing
If I'm ramping, I want the most mana for my buck, so now I'm looking at Worn Powerstones and possibly even Hedron Archives, skipping the possibility of getting my Commander early in order to get more mana for bigger stuff
Like most Magic: The Gathering cards, the value of Gravedigger depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Amonkhet)
Also of note, a legendary vehicle called the Weatherlight is here as Tornado, Sonic's Biplane, and thanks to a recent rules change, you could actually have that be your Commander if you are so inclined