OK, am I missing something? I can find the table for spending threats and despairs on magic checks, but not one for using advantages and triumphs.

What are people doing for these?

Extend range? Extend duration? Increase effects?

Pretty sure the intent is to use the "normal" table for spending advantages and triumphs. They just try to illustrate in the threat despair table that the risks of using magic should be greater than that of normal actions.

Note: I would personally advise against having advantage replicate a spell effect earned by a difficulty increase. Otherwise why pay the cost of having a more difficult check to increase range if you could otherwise just spend extra advantage for it.

Even letting triumphs replicate that gets a bit tricky, as it still seems better to then chose an optimal "easy" spell that can still be useful, and bank on rolling a triumph or two (when rolling 4+y, its not that uncommon) to boost the effect.

It should also be noted these tables are really more glorified suggestions. You are -never- fixed to just those potential outcomes for despiar/threat/triumph/advantage. What matters most is that the result is narratively interesting, incrementally stronger/weaker than having rolled less/more of the result, and agreeably fair at the table.

Oh, I LOVE the wild card narrative roles on magic.

It's magic. Narrator's McGuffin!

Fireball with a Triumph? Not only did make the bad guy go boom, it reduced his body to inorganic ash. They can't get DNA if they tried!
Forcefield with an Advantage? The way you move and bend the shield to protect you also starts shooting out the lights around you, giving you better cover.

...and it's even MORE fun when they start to get the evil die results.

We found this out the other night. Double Triumph on a Barrier spell to provide an armour to a character. We made it permanent until the end of the encounter (one would be to double the duration and the second until the end of the encounter).