First, be sure your variable is a "Number" type.

Then, when composing "Body" of your e-mail click the "Advanced Lookup" link, then choose your variable, and after it is chosen click the little "fx" icon next to its name. On the popup pane choose "Formatted Currency".

It should format the number according to the regional settings of your tenant (I hope so...), but I think the "$" character you must add manually.

Thanks Tomasz.  That's definitely helpful.

What I failed to mention is that I'm trying to build a HTML string in my workflow that displays the formatted currency column inside of a column in a table row.  That string will then appear in the email.  So something like:

<tr><td>Total Cost</td><td>{Variable:numTotalCost}</td></tr> 

But even though you should be able to insert variable using advanced lookup option, where you can choose format of the inserted value. Have you tried that? 

‌ does my answer helps?

I tried this and it worked.  Thanks Tomasz!