There isn't one. You could start with how many gems will get you a 3 card booster pack, but even then it really depends on demand. Especially since the cost to craft a booster with gems is directly tied to how many cards are in the set. In many cases, normal cards cost so little in the marketplace it's cheaper and easier to just buy them then try to trade them for gems.

It's only when you get to cards that can't be sold in the marketplace (delisted games), where trading for gems becomes a thing. And at that point it's really how much other people are willing to negotiate for.

Digital items are worth whatever someone will pay. Some of my trading cards are worth 25 cents because that's what someone is willing to pay for them. Others are only worth 3 cents. I have a foil card that I say is worth $3, but nobody else has been willing to buy it for a month, even though that's the lowest price on the market. So, I guess maybe it's actually worth $2.50. We'll see.