Gold piece to dollar exchange doesn't work well, D&D economy is all kinds of nonsense.
Current gold price per ounce is roughly 1200, 1 lb of gold in D&D is 50gp, so (1200 * 16) = 19,200, 19,200/50 = 384 dollars per gold piece.
But a loaf of bread is 2cp, and about 1 dollar at the store, so 5 loaves is a silver, 50 loaves is a gold, so 1 gold piece = 50 dollars.
You can't start comparing swords and things because are current society doesn't see them as tools as much as decoration and art.
I disagree.
I think that a 1 GP = $100 ratio tends to give people a fair idea of what a gold piece in the game is all about.
Non-magic weapons in the game typically range from 1 to 50 GP or $100 to $5000 equivalent. Guns in the real world tend to typically range in the $200 to $1500 range.
The majority of starting PC armor in the game typically range from 5 to 50 GP range or $500 to $5000 equivalent. Bullet proof vests in the real world are in the $500 to $1500 range.
Backpack $200
Blanket $50
Clothes $50 (common) to $1500 (fine, or high end suit in the real world)
Transportation: Horse $5,000 to $40,000, Car $15,000 to $40,000
Lifestyle: 1 SP squalid a day to 10 GP aristocratic a day ($10 to $1000)
Meals: 3 CP to 2 GP, 3 SP modest ($3 to $200, $30 modest)
Inn: 7 CP to 4 GP a night, 5 SP modest ($7 to $400, $50 modest)
Mug of Ale: 4 CP ($4)
Your loaf of bread example: 2 CP = $2.
Granted, a sheet of paper cannot be mass manufactured in a game world, that is why it costs $20 a sheet.
Is this a perfect 1 to 1 correspondence? No. But as a rough rule of thumb, the 1 GP = $100 ratio works well to give DMs and players a rough idea of how much money is actually being found or given.
If an NPC pays the PCs 50 GP, it doesn't sound like that huge of an amount. But, it's about $5000. For 5 PCs, that 's $1000 each.
That plate armor that protects so well and the low level fighter drools over is 1500 GP. That's $150,000. That's incredibly expensive.
But if one looks at "what are the general costs of goods and services", 50 GP in the game world will buy the PCs about the same amount of meals or nights in the inn or weapons as $5000 will in the real world.
This means that the Potions of Healing cost $5000 each. And if a PC finds one and can sell it for 60% or 30 GP ($3000), yes, he can live in a modest inn (5 SP) with 3 modest meals (3 SP each) a day (total 14 SP or $140 a day) for 5 weeks. Selling one set of plate armor for 1000 GP? That's 3 years of living modestly in the game world paying at inn rates (obviously, a person would find a boarding house and live 6 years off of 1000 GP).
It doesn't have to be perfect, it just has to be good enough so that players know what a 1000 GP hoard is effectively worth in terms that they understand ($100,000, that's some serious coin).
So in 4E, that simple +1 sword at 360 GP was $36,000.
That 30th level item in 4E? 3,125,000 GP or $312,500,000. Almost one third of a billion dollars. But, think about that. The wealthiest people in the real world are worth about 10 to 80 billion and can afford multiple toys worth one third of a billion dollars each. 30th level 4E PCs would be some of the wealthiest people in a fantasy world.
The thing that people have problems with is that they see that Potion of Healing at $5000 and say "I wouldn't pay that in the real world, so your 1 GP to $100 ratio must be wrong". Maybe you wouldn't pay that because players in the real world tend not to be adventurers who find and blow through hundreds of gold pieces like there is no tomorrow (which there sometimes isn't in the game). But many goods and services in the PHB make reasonable sense with this ratio. Perfect? No.