Reddit mentions of 1 LB Organic Wheat Seeds (Hard Red Winter Wheat)

Say one pound of wheat in DnD-land goes for 1 copper (0,01 gold). That number is based on this (and incidentally it seems to correspond roughly to the price of basic units of alcoholic beverage, probably the most commonly sold commodity in RollPlay).

Now, a pound of wheat in our world goes for between 6,99 and 3,50 on Amazon. Split the difference, say 5,245 USD for a pound of wheat.

If so, judging by wheat, the conversion rate is roughly 0,2 copper (or 0,002 gold) to a dollar.

Say full plate armor costs 1500 gold (as someone else suggested).

If so, the price for it would in our world be 750 000 USD (relative to the price of wheat we just considered).

However, that figure is more or less worthless because the relative value of armor and a pound of wheat would be a lot different than today. It is not a given that we base the conversion rate on the price of a pound of wheat. We can base it on the price of armor, if we find some equivalent to armor today.

In order to do that, we have to consider the general qualities of full plate armor in the world of RollPlay. It is not like a car -- a car, in the West, is something owned by most people. Unlike wheat (and wheat-based products), plate armor can't be subject to (an equivalent of) mass production in RollPlay. It needs individual specifications and a long production time, with hard gotten materials.

Furthermore, full plate armor is an object of social value, designating a certain class (alluding to knighthood). If owning armor at all is like owning a car, full plate armor is like a very luxurious line of cars.

In that context we can see how it costs as much as it does, as luxurious types of cars going for 750 000 USD -- while excessive, not inconceivable.