Extras

[BronyHeresy]

How much would you say an attack is worth in magic points? How many magic points would you be willing to spend to attack with one extra unit per turn? How much would you say a move is worth in magic points? How many magic points would you exchange for an extra move, or to move a unit extra spaces? What about giving up movement or attacks for the sake of getting extra magic? This can apply to any unit or faction, not just ones that have extra move/attack abilities. Tundle, for example, sets the possible precedent, that 2/3 regular attacks are worth 1 Magic.

[Scapemage921]

I think 1 magic point per extra attack or move is the price I'd be willing to pay. In a pinch, an extra attack or move can be life or death.

[Quintaton16]

A horse, a horse! My kingdom for a horse!

This depends so much on what the board position is like. In most cases, there aren't more than 3 legal attacks, so an additional one is entirely worthless. When it can be set up one extra attack would often net you one extra kill, making up the Magic point, so I would always pay 1 Magic for an attack. From a game theory perspective, that indicates that the price is far too low, and that adding attacks is worth more that 1 Magic each. However, Tundle's ability is fairly costed. Since there are turns where extra attacks are completely wasted, gaining even a small amount of Magic, like 1 Magic per two attacks, is a complete no-brainer.

As in the quote above, there is no fixed exchange rate. Having four attacks one turn and two the turn after that isn't even close to an even trade in any situation that you would agree to make the trade in.

How much I'd pay to move an extra Unit is dependent entirely on how easily my opponent can buy an extra attack to kill the Unit I moved.

[PePe QuiCoSE]

as a comparison: disregarding board state I would rarely pay for Shadow Arrows if it's to attack another common but will almost always do if it's to attack a Champion or a Summoner (in part due to SE nature but also because Commons are replaceable).
My point of start when thinking about this is that putting the last wound on the enemy summoner is worth everything I can pay for it. But putting SOME wounds on it depends on faction/board state. Note that the Theory Exercise I posted some time ago deal with something similar, how much would you spend to do 2 automatic wounds on an enemy Summoner.
Falling back again to existing units, CG Fighters seem to indicate having an extra attack is worth 1 Magic (their own card) and cards like Shikwa and Quen also indicate that attacking again with one unit is worth 2 (points) Magic respectively.