Fixing SKR's Feat Point System

[Airanath]

And even if you are focusing on a single school, there are usually better feats. +1 to the DC is nice, but you can achieve that in other ways. Spell Focus is mostly a requisite for some Prestige Classes. It might be a bit low at 5, but I think 7 is too high, it lacks versatility, and is situational, more than, for instance, any of the metamagic feats (even without cost reducers).
I would say 8-10 for sure if it actually lowered metamagic costs for the foci school.
Also, if you are going to take it just for the DC increase, you are also going to take Greater Spell Focus, which will increase its price to proper levels for +2 d/c (That would certainly be a 10 as a pair). With that in mind, 6 for spell focus is what I would recomend, maybe 7, and lower Greater by one point in that case. Either way, it costs 11 points for the combo.

Edit: Also, the problem with DCs lies in how spellcasting DCs progress, they are a static number based on spell level. Look at psionics, where you can invest extra power points (use higher spell slots), to get the benefit of an increased dc. You can still use Psionic Endowment(and Greater) to raise the DCs more, but they usually progress pretty well with your levels. Spell focus isn't cheap/powerfull, Heighten Spell is the silly feat in there.