On the specific example of the spell:
Sense Foes - Information; Area; Info
Tells the caster if the subject has hostile intent, and what the degree of hostility is. Can be cast on one person or a whole area. If cast over an area, this spell only detects that someone is hostile, without telling who.
Base cost: 1 (minimum 2).
[Source: GURPS 4th Edition Basic Set - Characters p.245 or GURPS Magic p.45]
The section on Area Spells (B239 or M11) explains this minimum as:
The cost listed for an Area spell is always its “base cost.” The actual cost to cast the spell is equal to its base cost multiplied by the radius of the area of effect in yards (minimum one yard): base cost x1 for a one-yard radius, x2 for a two-yard radius, x3 for a threeyard radius, and so on.
Area spells with a fractional base cost, such as 1/2 or 1/10, cost a minimum of one energy point. A few Area spells specify a minimum cost, which you must always pay, even if it is larger than the base cost multiplied by the desired radius.
If I have a high enough skill in Sense Foes to generally be eligible for reduction to casting cost (B237 or M8), does that apply to the minimum as well?
[...] the cost reduction for high skill only applies to the total cost of the spell.
Sense Foes cast on an area of 1 yard radius calculates at 1FP,
but is raised to 2FP because of the minimum.
Do i get those 2FP waived if I have a skill of 20 or above?
Sense Foes cast on an area of 3 yards radius calculates at 3FP,
not increased because i't above the minimum.
Will skill 20 reduce my cost to 2FP (the minimum) or to 1FP (the standard high skill bonus)?