What I think makes these priced as they are is that they all give disadvantage on saves against the bard's spells that cause the Charmed condition, which is indeed very powerful and worth more than the spel-llike abilities the instruments give. IMC I have introduced much cheaper versions that lack this one ability.
Yes, but that is a flaw of assumption!
Sure, they are strong, but
some of them are common.
I mean, lets take D&D. Lets remove
all weapons. What is the value of a greatsword? It adds 6 damage per attack to every melee PC! That is a crazy strong item.
Or remove shields. What is the value of a shield? An item that adds +2 to AC? Doesn't even require attunement? Amazing!
Or remove all armor, then price plate armor -- +8 to AC! CRAZY! That is stronger than any defensive item in the entire game!
The power of a magic item is a
ceiling on how much it costs. The floor is set both by alternatives and by demand. Here, the authors of the list chose the ceiling; elsewhere, they chose the floor.
When they note that winged boots are better than another rarer flying item, here we have direct competition of items - and you can argue that if winged boots are easily available for 500 gp, then the inferior alternative shouldn't cost more.
(This argument only holds if you assume items are available on-demand for the prices described. If there is significant friction in finding specific items for sale, price inversion is perfectly reasonable; the price of flying items floor is not "does a better item sometimes get sold for less" but rather "do I need a flying item right now, and can I find a cheaper one right now, and how much am I willing to wait for finding a cheaper one").
But the instruments of the Bards have no such comparables except each other. You can't compare them to a warlock focus, because they aren't usable by the same people or spells.
If we presume the DMG treasure tables and the word "uncommon" says what it means, then we also know how often they show up. And as written, the insanely expensive uncommon items they have in the price guide mean a lucky roll makes the PCs insanely rich and able to buy really crazy items (like cloaks of invisibility).
The prices for magic items in the DMG/XG assume a low magic setting where there is no "magic item economy" to speak of. Buying a magic item is like trying to buy something unique, like the Mona Lisa. Prices are huge, pretty random, and you are generally going to have to deal with shady people to get hold of it.
This is consistent with the 5e rules being balanced around parties with zero magic items. It is not a good fit for higher magic settings like Eberron (which was notably designed for 3rd edition) where you can reasonably expect to walk into a magic shop and pick something off the shelf. Under these circumstances completely different price scales are required.
Sure. But in Eberron, what I'd do is work out what items are commonly for sale. The prices will be set by the various manufacturing guides.
They'd even be tempted to do annoying things with attunement magic items. Imagine magic items whose attunement can only be changed by members of the guide? That would be a fun curse, and would shut down used magic item sales.
The "standard" magic items will mostly be tools.
Expensive combat magic items will be status things. And as such shouldn't be optimized for efficiency, but rather for flashiness.
There will be a tier of moderately expensive enhanced gear. Elven Chain Shirts for discrete protection, Mithril Armor for bodyguards able to move faster. But mass equipping soldiers with +1 gear is uneconomical; you'd rather have your enchanters build firearms, things that change capabilities instead of making existing skills slightly better.
OTOH, we could have all militaries using +1 gear. The gear is branded with sigil of the military in question, and use outside of that military is proof of theft from the state. So now we have "cheap" but dangerous black market +1 items, possession of which is a capital offence.
Lots of fun stuff.
The issue with 5e is one of gold sinks. If you can't spend your loot on magic items or castles, what are you going to do with it? But this reflects a general move to story based rewards rather than wealth.
Sure, but you can treat gold as a story based reward instead of an adventuring gear based reward.
Even in a magic-heavy world, not all magic items need to be common. You get to pick what.
But what I'm trying to say is, I would actively attempt to avoid using gp as a secondary xp system. It didn't work that well in 3e and in 3e they tried to design the game around it. It didn't work that well in 4e (although a tad bit better), and again the economy and part of gameplay was designed around it. 5e wrote it off.
If you want to reintroduce gp-as-advancement I'd go back to the drawing board. Maybe steal from 4e.
The result will be a different magic item list, a different power curve, and a bunch of other issues. A simple price list ... doesn't do a good job. Especially one without a pile of work the SMIP people didn't seem to do.