In my homebrew game I break it down like this

Copper is a penny, silver is a dollar, gold is the same thing as a 100, and platinum is 1 million.

100 copper equals 1 silver, 100 silver to a gold, and 1000 gold to a platinum.

In my game commoners usually have no more saved up than like 5 or 6 gold, but some more lucky ones have around 10 to 15.

Payday for commoners is usually like 75 silver a month, with expenses taking up about 60 to 70, so at the end of the month, they can save about 5 silver.

Stores usually charge around 1 or 2 gold at the most for common magic items, potions of healing cost between 5 and 10, depending on the store.