As written, when you create a Missile spell such as Lightning, Fireball, Stone Missile, etc. you get the energy savings discount on the total cost, not per turn. Per the GURPS FAQ:
4.3.3 It seems missile spells are much more powerful than before. Is that right?(Italics mine)
Yes. Now you can put up to [Magery * base cost of spell] energy each second in it, for one to three seconds; spell effect is multiplied by each multiple of the base cost. Note that the cost reduction for high skill only applies to the total cost of the spell. With skill 20, you can't cast a 6d fireball for free by concentrating for 3 seconds.
Short version? In Felltower, that last sentence is wrong. You absolutely can cast a 6d fireball for free thanks to your skill 20 by concentrating for 3 seconds.
Honestly, I didn't know this was the wrong way to do it originally. But even once I did, I found I preferred the way this made Missile spells a cheaper way to apply damage to a target than most other spells. They're slower (up to 3 turns of casting, and at least one turn of attacking) and you need to hit with your throw, as well. You need a free hand. Targets can Dodge or Block and those are usually fairly effective defenses in a low-tech world with missile weapons to deal with.
So if you wonder why the players in Felltower favor 6d missiles so much, it's because of this ruling. Ggetting to skill 20 isn't difficult. It only takes IQ 16 and Magery 6 to have it with all spells for 1 point, and DF Wizards start with IQ 15 and Magery 3 and enough discretionary points to get to Magery 6 immediately.
And I like it this way. Bring on the high-damage Missile spells!