I think a lot of the obvious points have been made: pricing has to be
relative to level; it's hard to build in the effects of cumulation/stacking, but these are pretty crucial for balance; etc.
Elaborating on another point that's been made: +1 leather is not a heap more valuable than studded leather, and so probably shouldn't cost the same as getting +1 studded, or +1 plate, which allows breaking out of the limits of the armour chart.
The same sort of thing goes for weapons, too. Compared to a +0 longsword, a +1 longsword grants (say) about +1.5 to expected damage per attack (say your base chance to hit is 7/10, for 1d8+4; well now it's 3/4, for 1d8+5). But a +1 dagger, compared to +0, grants only about +1 to expected damage per attack (7/10 for 1d4+4 becomes 3/4 for 1d4+5). (The dagger's expected damage
multiplier is about 11/9, compared to 5/4 for the longsword - pretty comparable, but working on a smaller base.)
That said, some weapons are more likely to faciliate landing other damage bonuses (eg sneak attack) which makes the maths harder and increases their desirability.
If you're going for a price list you want constant rather than purchaser-relative prices. But I think you do want to think about optimal uses - so daggers and shortswords need to be priced to reflect a thief's interest in them, rather than the quirky dagger-fighting paladin. And with items that can stack, evaluate the higher-level ones (eg same bonus but no attunement required, or no encumbrance, or whatever else signals them as higher level) on the basis that they are stacking with the lower-level ones, and hence price them at
even higher level.
We should not allow level 9 scrolls on the cheap - what's the hard limit on 1 spell per day worth if all you need to do is unload a few tens of thousands of gold on scrolls?
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In fact, scrolls need to be fundamentally more expensive than in 3rd edition (and therefore Sane). The edition is after all built upon "no magic items needed", so infusing dozen of scrolls throws everything out of whack.
The same probably goes for potions, wands etc that duplicate spell effects. (Another consideration that points in the same direction: in 5e the AD&D function of attack spell wands (giving MUs something to do) is replaced by cantrips.)
On the other hand, the DMG assumption that a consumable should reasonably cost half of the equivalent permanent item is of course completely bonkers. Utility, folk! When you feature magic shops you give players a choice (that's the whole point!), and no player will ever pay X for a single-use item when a permanent use-as-much-as-you-like item costs 2X.
I think this depends a lot on what the item is.
If the consumable is 100 gp, and the permanent item is 200 gp, and the party has (say) 5,000 gp to spend, then I agree with you - they'll buy the permanent.
But if the party is poor (= low level), and needs the item for one particular purpose, then the consumable can become quite attractive - eg they need a PC to be able to breathe underwater for one particular heist; or they need to power up the rogue with magic to get one good sneak attack against the wererat; or whatever it is.
To try and abstract out a general principle: if consumables are cheap compared to permanent items, they become attractive to players of higher level (= richer) PCs trying to build a grab-bag of capabilities; if they're relatively expensive (eg half price), then they're attractive mostly to player of (relatively) lower level PCs wanting to boost their capabilities for some particular focused purpose.
Either seems a viable approach, but maybe the second is closer to the default spirit of 5e?
As the DM you determine how much gold the party gets in any given encounter. As the GM you also set the prices of items (especially ones with no normal list price). So really we are setting an exchange rate of encounters to magic items.
This is more-or-less how 4e handled it, although mediated through the treasure parcel system.
