Like most Pokmon cards, the value of Diglett 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 (Expedition Base Set)
You get to draw an extra card, re-spend all your mana, and get an extra combat stepnot to mention extra upkeep/end step triggers, planeswalker activations, and so on
While it's one of the top five decks in Standard, it doesn't seem like there's one clear, settled 'list' for Golgari Midrange right now
A couple of standouts are Teferi, Master of Time who will quickly become BFFs with Sefris given a chance, and Enduring Renewal, which we can play out once we have a creature or two in play and ensure we draw straight through to our lands/noncreature interaction every turn thereafter
we will update as soon as we can confirm