Adding a dollar sign to slider expression creates an error.

Martin and Mylenium are correct.  It sounds like you've put the expression on the slider instead of the text.

I'm assuming you're just trying to add a $ sign to the front of your slider number, round the number, then put the result into a text layer:

Get rid of your existing expressions.  Then add this expression to the "Source Text" parameter of your text layer:

"$" + effect("Slider Control")(1).value.toFixed(0)

As Martin indicated, you may simply be using the expression  wrong. More info on what you actually do would certainly help, such as a screenshot of your timeline with the expression...

Mylenium

A slider can only take numeric values and "$" is a string.

Not sure what you are doing, but if you want "12$" as a text, where 12 is the input from a slider, you need to put this expression to the text, and not to the slider.

*Martin