Something like this may have been done before, but I'm being lazy here and not looking though Thaumatology to see. Well, I looked through it but I didn't examine it in depth to see if this is offered up.
Magic Is Never Free
- Use GURPS Magic as written (or as modified).
- No spells are free. Minimum cost to cast is 1, after all other modifications, minimum cost to maintain is 1, after all modifications.
- No penalty for spells "on."
In short, spells are never free to maintain, or free to cast. Spells already cast don't interfere with spells you want to cast.
Options and Comments
No penalty for spells on means there isn't a limit on the number of spells you can cast due to chances of failure. In play, the spells on penalty is critical - spells that don't give it can be abused freely (Create Servant), and spells that do are capped only by the increasing difficulty of getting a further spell off.
FP and Energy Reserve are more important than skill. Skill reduces but doesn't eliminate cost, and it increases the range and effectiveness of spells. But the limit on casting is FP, not skill. Skill is mostly to overcome distance and resistance, not to overcome cost (even though it does indeed help).
You could allow a Perk that let a single spell be 0 to cast if you can otherwise reduce its cost to 0 via skill (For example, a spell that is 1 to maintain and skill 15.) I wouldn't allow 0 to maintain, personally, and absolutely not in combination with 0 to cast or you're back to where you started. A 0 to maintain spell, coupled with no spells on penalty, is defeating the entire approach. However, allowing a spell to be free at sufficiently high skill + low power levels would allow casters who can fling 1d-2d Fireballs around all day, or ignite candles with a snap without effort, etc. or do other things without allowing for the "I Levitate around Invisible all day with Dark Vision and See Secrets on" guy.
As it stands now in my game, skill is king until skill 20 or so, then FP/Energy Reserve is king. The goal is to get a lot of spells you can zip off for free and maintain for free, then have enough skill to get off spells with a -5 to -8 on for spells "on." I think this change would allow for greater effective combat use of spells (harder to resist, easier to get spells off on allies) at the cost of the need for lots and lots of FP.
I know the usual alternatives are "Don't allow skill to reduce cost" or "All it to reduce cost at a penalty to skill rolls." But I feel this is a sufficiently different approach to give a different flavor to a gave using it instead of those alternatives.