Card prices for Frontier are starting to move, as Jace, Vryn’s Prodigy has gained $10 in value recently. People are taking an interest in the format, and popularity sparks demand. For the most part, mythics are going to be the cards to watch, though it is worth noting that Frontier is the only format where you can currently play Dig Through Time.
The card has gone from trending towards the bottom of the bulk bin to being about $1.50. There are still over 200 listings on TCGPlayer and plenty of copies around, though if you peruse the decks that people are playing in Frontier, Dig Through Time is seeing plenty of play in the format. Demand will increase as the format gains popularity, assuming that my confidence in Frontier isn’t misplace.
Given the fact that you can only play Dig in Frontier, the ceiling on this regular rare is going to be pretty low. I would also caution you that as the format gets larger, delve is one of few degenerate mechanics that is Frontier legal, which gives me pause with regard to the card and a potential ban down the line. At any rate, if you’re looking to play Dig in Frontier, now is good time to pick up your set, though I wouldn’t advise going too much deeper.