Metamagic feats that reduce required spell slots

[SinsI]

If any of these feats would reduce the required spell slot to lower than 1/2 of the actual spell level (rounded down), they reduce it to that level (1/2 of actual level, rounded down) instead. (0-th level spells stay 0-th level)

Shrink Spell (Metamagic)
Benefit:
You can alter a spell with a range of short, medium or long to reduce its range. A shrunk spell that initially had a short range (50 ft + 5 ft./level) has a range of close, medium-range spell (range of 200 ft. + 20 ft./level) becomes short range and long-range spells (800 ft. + 80 ft./level) become medium range. Shrank spell uses up a spell slot one level lower than the spell’s actual level.
If applied together with Enlarge Spell, these feats cancel each other.

Shorten Spell (Metamagic)
Benefit: You can reduce the duration of a spell with a duration of longer than 1 round by 50%. Shortened spell uses up a spell slot one level lower than the spell’s actual level.

Decimate Spell (Metamagic)
Benefit: You can alter the duration of a spell with a duration of 1 minute or longer. Decimated spell has its duration reduced by a factor of 10.
Decimated spell uses up a spell slot 2 levels lower than the spell's actual level.
Special: You can apply this feat to the same spell multiple times, as long as the final duration is at least 1 round. Each use of this feat after the first reduces required spell slot level by 1 instead of 2.

Minimize Spell (Metamagic)
Benefit: All variable, numeric effects of a spell modified by this feat are minimized. Saving throws and opposed rolls are not affected. You can only apply this metamagic to spells with random variables. A minimized spell uses up a spell slot three levels lower than the spell’s actual level.
If applied together with Maximize Spell, these feats cancel each other.

[jiriku]

Let's see:
Cast minimized summon monster VI to summon 1d3 5th level monsters. Get one 5th level monster (the equivalent of casting summon monster V) for a 3rd level spell slot.
Cast doubly decimated shortened forcecage and imprison an enemy in a cage of force for 7.8 minutes for a 3rd level spell slot. Wall of force, by comparison, is 5th level and lasts 1 round/level.

This doesn't pass the smell test.

I think the trouble you have is that whenever anyone pays extra spell levels for metamagic, the cost is fixed, but the benefit could be as low as zero if you apply a metamagic feat to the "wrong kind" of spell. The everyone always uses it on the spells where the benefit is largest. However, if you have a negative spell level adjustment, the reverse is true: the benefit is fixed, but the cost varies. For example, reducing range on a noncombat spell for which you can easily approach the target is no cost at all. Likewise, reducing duration on a spell that lasted much longer than you needed costs nothing at all. In these situation, the player receives a fixed benefit for little or no cost. I don't think there's a way you can even hope to balance this.

[SinsI]

What if you can't use those spells to reduce spell slot required below original?
This way you can use them to make other metamagic cheaper.

[jiriku]

That would put you in a much better position, yes. Paying, say, +4 levels for quicken and then getting that +4 adjustment refunded by cutting your range and duration down to the bone is reasonable (spells like swift fly or swift invisibility do that, for example). Ok, yeah, that could work pretty well.