looking for a rule/feat

[death390]

in dnd 3.5 i vaguely remember a rule or feat that allowed a focus to replace a expensive material component. so for example you could replace the 100g cost for identify with a more expensive focus that you dont have to pay for more than 1 time. this is obviously only useful for spells which would get cast repeatedly but so expensive that you have to wait mid/late levels to get access to the wealth necessary to use it.

its not craft rune, that replaces inexpensive components with a rune focus costing 1g per spell.
nor is it eschew materials

and the actual arcane focus feat from dragon magazine is a psionic focus knock off.

[Uncle Pine]

There is Arcane Focus Item for cheap material components, from Dragon #358:

You can craft a unique signature staff or other item that serves as a focus for your magic.
Prerequisite: Craft Wondrous Item, any metamagic feat, arcane caster level 3rd.
Benefit: You create a personal arcane device that you carry with you, typically a staff but optionally another item like a pocket watch or amulet. This device serves as a focus for magical energy, improving your ability to apply metamagic effects to spells. You must hold the item in your hand in order to gain its benefits. If you spontaneously cast spells (as a sorcerer does), three times per day the focus allows you to apply a metamagic feat to a spell without increasing its casting time. If you prepare spells (as a wizard does), three times per day you may spontaneously convert any spell into a metamagic-enhanced version of any lower-level spell you currently have prepared. Converting a spell in this way forces you to lose the higher level spell, which must be of the appropriate spell level for the spell you cast and the metamagic feat you use. You must have the metamagic feat you wish to use in this way. For example, you may spontaneously lose a prepared fireball spell in order to cast an empowered magic missile, but only if you have both a normal fireball and a normal magic missile spell prepared. In addition, you may treat the item as an arcane focus in place of any spell components worth 1 gp or less, and a hand in which you hold the device is considered your free hand for the purpose of casting somatic components. You may also use magic item creation feats such as Craft Magic Arms and Armor, Craft Staff, and Craft Wondrous Item to enhance the device as an item of its type. If you lose your arcane device or it is not on you, all your spells suffer a -1 penalty to caster level until you recover it or create a new one. If you have your device on you but aren't wearing or wielding it actively, you suffer no ill effects but neither do you gain any benefits.
Creating an arcane device costs 100 gp and takes 24 hours.
Special: A wizard may take Arcane Focus Item as a wizard bonus feat.

I could not find any other feat resembling what you're looking for in 3.5e, but PF has False Focus (https://www.d20pfsrd.com/feats/general-feats/false-focus/), as well as Blood Money (http://www.d20pfsrd.com/magic/all-spells/b/blood-money).

[Zaq]

I can’t recall the source either, but your description triggers my “that sounds vaguely familiar” feeling as well.

Perhaps it had something to do with turning spells into SLAs or into items?

Thinking about it critically, it does seem somewhat against WotC’s style to print such a thing, since slapping expensive components on something was a big way for them to attempt to impose “balance” on something (with rather mixed results, mind you). I could see WotC getting very skittish about intentionally allowing that rule to be worked around. That said, there’s an awful lot of printed material out there, and it’s not like they never broke any unwritten rule patterns before.

[death390]

closest thing i remember is the 50x cost for magic item creation of spells with costly components, which is what i think it was based on. but yeah that vaugely familiar feeling is off.

[Uncle Pine]

closest thing i remember is the 50x cost for magic item creation of spells with costly components, which is what i think it was based on. but yeah that vaugely familiar feeling is off.

What bugs me is that I also have the same vague remembrance... But off the top of my head I can only think of Runesmith's permanent rune, which does exactly what you describe.