How much is a Gold Piece really worth?

I have been wondering what a gold piece is worth, due to the fact there are many things that cost the same, but in reality, are probably worth radically different prices, such as a Goat and a Whip, which both cost 1 gp.

What is a gold piece really supposed to be worth? Like, how would things be priced if they were in familiar modern monetary units instead of “gp”? That would give me something to base my adjustments to abnormal prices on, such as items not listed in the PHB.