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)
Tataru Taru At two mana, Tataru Taru plays an incredible role in making a drawback for any opponent that draws cards outside their turn
Worst case, your Mindspring Merfolks exhaust ability gives it a +1/+1 counter and lets you draw some cards
Please make sure to check your Tidings before you buy
Its an interesting ramp payoff in a deck filled with big creatures
Also, you can eat your Grief if the opponent tries to exile it, so it's safe in your graveyard to be returned later with Priest of Fell Rites