Try $\frac{\$100.00}{\$10.00}$
, which is the usual TeX approach to handling dollar signs within an expression. If that doesn't work for you, then you will have to give more details about the situation where you are using MathJax (such as your MathJax configuration block, and whether you are using a content-management system to enter your text, etc.).
As for the font issue, you will need to give more details about what. MathJax is only designed to work with the fonts that is comes with, and it has the bounding box information only for those fonts. MathJax doesn't have the bounding box information for other fonts, so it will have to guess about that, and likely gets it wrong.