While "$10 USD" may be redundant in a newspaper published in the USA, it's immensely useful and arguably preferable when writing blog posts, emails and other text destined for the "World Wide" Web. While USD is commonly used as and many are comfortable with its use as a "common denominator" when pricing something on the Internet, it's still very important to be clear "what dollars do you mean" in this context.
I used to do "10 USD" or "USD 10" until I got sick of hearing responses like
"USD 10 looks weird, why did you do that', or 'that on the pricing page looks funny, can you fix it up a bit'
It seems $ (or the equivalent currency symbol for other currencies) has a place in many peoples minds implying that the number it is next to is currency, and they seem to find it weird when things involving currency are 'written correctly' without the symbol that the numbers mean currency.
While "$10 USD" may be redundant in a newspaper published in the USA, it's immensely useful and arguably preferable when writing blog posts, emails and other text destined for the "World Wide" Web. While USD is commonly used as and many are comfortable with its use as a "common denominator" when pricing something on the Internet, it's still very important to be clear "what dollars do you mean" in this context.