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.