The formatting is MathJax and is pretty useful for complex math. If you want to use a normal dollar sign you have to "escape" the symbol. This means that you tell the interpreter "You know, the following is just a normal character. Not some formatting stuff like 'I want this italic', or 'I want this bold' or 'I want this in fancy math-whatever style'".
To escape the dollar sign simply use a backslash in front of it.
\$ will produce $.
I have \$10. With my \$10 I am going to buy apples. produces
I have \$10. With my \$10 I am going to buy apples.
As I said in the text this can also be useful to turn off other formatting stuff. For example
\* to get * instead of italic. This also means you need to write
\\ to get \, as
\ is the special character for escaping special characters.
For more examples about using MathJax you can also look on Meta How do I add mathematical notation using Latex MathJax?.