I feel like it's important to at least mention whether a piece of sofware is free or not. Including the price seems to make a lot of sense. When possible, I think it's best to provide a primary source to back up your stated price, even if that primary source is the vendor's store.
There are several reasons for why I feel a primary source is best - and almost all of them involve potential obsolescence:
- If the price changes, the link to the vendor's site (or store) will likely be updated too.
- If you link to a secondary source (Blog post, forum post), the link may die or the information may be incomplete.
As far as Currency (but not "USD vs $" syntax) is handled, I say that using USD as a standard is the best way to go, when the asker hasn't specifically mentioned the currency they use, or if the software is not sold on a market that accepts USD. This is because
- The most common country for companies to distribute their software is likely the U.S.
- A large portion of the site's userbase will be U.S.-based.
- Users who are not U.S.-based will at least be able to get an idea of the cost of a product in their local currency because USD can easily be compared with most others (But this doesn't account for import taxes, etc)
The syntax argument ($ vs USD) is valid, but it's not realistic to expect everyone to actually type "USD" instead of "$". (Although still pertinent, considering - for example - Canada's Dollar is also represented with the $ symbol).