We write in US English. So, unless you know the audience for your content is exclusively in a country outside the US, you should default to the short format (currency symbol plus numeric value) to refer to currency amounts ($1,200).
If you're writing for audiences who might be in countries outside the US or about currencies you think customers might not find familiar, use either the ISO code and currency value (CAD1,200 - no space between code and amount) or the explicit format of currency symbol, currency value, and ISO code ($1,200 CAD; €1,000 EUR; ¥1,200 JPY).
If you're writing about multiple currencies that use the $ symbol, use the appropriate three-letter currency code (USD, CAD, AUD, SGD) after the number (They spent $4 million USD, or $5.1 million CAD, on the new platform) to disambiguate the types of currency.
Don't add a space between the symbol and the amount.
Use a period (decimal point) to indicate cents and a comma to separate thousands ($1,500.50). Note that there are no cents in Japanese currency.
Never use the cent sign (¢). Instead, spell out the word cents and use numerals for amounts less than a dollar (5 cents, 25 cents).
For the word euros, spell out lower case unless it starts a sentence.
Editor's note: This guidance is for people writing in English. Writers in other markets should refer to the relevant style guidance for their local country.