How should references to amounts of money be styled? The key to answering that question is context.
Casual, isolated references, as well as approximations, are generally spelled out, as in the examples below:
“I’m just putting in my two cents’ worth.”
“Remember when a candy bar cost twenty-five cents?”
“I have only five dollars in my wallet.”
“She loaned him a thousand dollars.”
“I wouldn’t trade it for a million dollars.”
However, in financial contexts—those involving anything from numerous references to salaries of specific athletes in a book about professional sports to pervasive citations in a book about business or economics, use the dollar sign and numerals: “He was the first professional football player to earn an annual salary of more than $1 million.”
Note that for round figures, using a pertinent figure followed by a term for an order of magnitude (such as million) is preferable to numeric notation ($1,000,000), unless the figure is precise, as in “The charitable event raised $1,567,893.” Generally, however, such precision is unnecessary, and an amount can be expressed with rounded-up approximation: “The charitable event raised $1.6 million.” (A casual reference can read “1.6 million dollars.”) In some financial contexts, K is used as a symbol for thousands, as in $125K in place of $125,000.
Dollar amounts are always spelled out when they begin a sentence, so if an amount is more or less precise, to avoid a cumbersome expression such as “One hundred twenty-five thousand dollars is the starting salary,” recast the sentence so that the sentence doesn’t begin with the figure: “The starting salary is $125,000.” (Beginning a sentence with the spelled-out version is acceptable for an isolated reference in a nonfinancial context but should be avoided when using dollar figures in abundance.)
Note that hyphens are employed when a spelled-out number is part of a phrasal adjective (“The five-million-dollar budget wasn’t enough for their ambitious plans”) but not with the dollar sign and numerals (“The $5 million budget wasn’t enough for their ambitious plans”).
In international contexts, the US dollar or an equivalent amount in another currency is usually indicated as shown in this example: “The US$6.5 billion aid package was part of the deal.”
The cent sign is rarely employed, except in cases such as indication of a printed cost, as on a price tag, in which case “seventy-nine cents,” for example, would be expressed “79¢.” In financial contexts, however, use a numeral and the word cents, as in the phrase “79 cents on the dollar.”