If the question is how can characters use their gold, a list of items with prices certainly seems to be advice on exactly that
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The intended design goal of 5E goes against the above. They don't want rules for everything. They don't want every group to play the game the same way.
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They expect the DM to have input on the game and how it works. And I think this also applies to the players, by implication. Come up with an idea...."Hey I bought some fancy clothes...can I gain advantage when I try to persuade the duke to help us?" and bring it to your DM rather then to the rule book.
A couple of initial things:
(1) They don't approach combat this way: "Hey, GM, I've got a claymore rather than a dagger - does that give me advantage on killing orcs?"
(2) They don't approach prayer and sorcery this way: "Hey, GM, I've got a holy symbol blessed by St Sigobert - will that give me an advantage to repel the vampire?"
That tells me something about their expectations - they expect everyone's games to have rather circumscribed fighting, prayer and sorcery (unsurprisingly, much like most editions of D&D since forever!).
Those discrete, different, detailed and tigjhly circumscribed systems also are the reason I can't take the idea of 5e as "rules light" seriously. I mean, yes if the comparison class is Hero and Magic Realm - but otherwise not remotely.
And this goes back to the issue of equipment lists: the game
doesn't just have an equipment list for swords and shields, for wands and bat guano. It has ultra-detailed rules for how these things factor into key activities of gameplay: a sword boosts your damage roll - a mechanical thing - in this precise way; a shield boosts your AC - a mechanical thing - in a precise way; a wand boosts your spell DC - a mechanical thing - in a precise way; having a pouch of bat guano and other knick-knacks opens up mechanical possibities that otherwise aren't there.
The fact that the game takes one design approach to fighting and casting spells (which themselves tend to have a strong focus on their use in fighting) and a different approach to dressing up in fine clothes to impress people tells me something about the game.
Could they have come up with rules for wardrobe and the impact it has on Persuasion or other social checks? Sure. Could they have come up with rules for how to build strongholds? Sure.
But they realize the importance of these things will vary from table to table. So they've left such things up to a specific group to decide.
But grappling, or conjuring prismatic spheres, or repelling vampires through prayer, is of equal importance at all tables?
This is why I don't think there will be any convincing you. I think it's a key aspect to the design. Not feeling the need to spend time and space around a bunch of areas of the game whose importance will vary drastically from game to game and committing a bunch of mechanics to those areas ahead of time.
In this discussion there's also a recurrent tndency to think that uniform resolution = 3E-style "rule for everything". But Cthulhu Dark has uniform resolution rules that fit on less than an A4 page. Prince Valiant has uniform resolution rules that fit on a couple of pages. HeroWars/Quest has uniform resolution rules that fit on about half-a-dozen pages.
Part of what makes 5e a rather complex system is its wide vareity of resolution subsystems that aren't straightforwardly integrated (eg deft finger moves to pull of stage magic may well invoke the skill/ability resolution system; deft finger moves to cast spells rarely do - they are a player-side fiat mechanic) but generally can't just be ignored (eg in Burning Wheel the sorcery subsystem can be ignored, and magic use resolved by a skill check - on the Sorcery skill - like anything else; in 5e there's no default generic mechanic that can be used in lieu of the magic subsystem).
I think that Moldvay Basic is basically a complete game - it puts itself forward as a dungeoncrawl game, and it has the mechanics to deal with that. I think that 5e is an incomplete game, in that it puts itself forward as covering a range of stuff for which its rules and mechanics are incomplete. Not because they need to be if it's to be kept "light", but because there are other design sensibilities at work - in particular, a preference for GM decision-making as to what happens in most cases that don't involve fighting or casting spells.
It's like a feature of classic D&D, which results from the extension of gameplay beyond the dungeoncrawling it was designed for, has been erected into a principle.
My game is not exactly the style you assume above, but my players' characters have indeed accumulated some money through play. They've used that money for a variety of things....most of which are more narrative than mechanical. They've established a trading company and they've needed funds for political purposes. There is upkeep involved in that, and a whole bevy of NPCs to pay for, and further investments related to the busines. One PC used the funds to establish a temple. Another used money to help in his search for his family.
What I find most striking about this is that you classify all this action as "not mechanical".
It seems to me based on your comments here, and in other conversations, that you want all mechanics to be determined ahead of time so that the players and DM have this established understanding of exactly what's possible and what works and how ahead of time. There's no judgment needed on the DM's part.
The last sentence seems pretty absurd in the context of a RPG: I don't think you can have a game in which fiction >> mechanics >> fiction without some sort of judgement being made, and in anything like a traditional RPG allocation of participant roles that will be the GM.
I just don't think that "the GM decides what happens" is a very interesting example of "light" design, especially when it's not implemented consistently (which it isn't in 5e - that's not the rule for resolving fighting in 5e).
I also think that "the GM decides what happens" isn't the best recipe for satisfactory play, but in the context of this discussion that's a secondary thing.