There’s a couple of options here. For the datagrid, you can add currency as a property for the object, fill it by default (or not, if applicable) and add a column before the amount without a caption – then use text-left and text-right to align the columns to show their contents next to eachother.
An alternative is to add a (fixed) currency via styling (CSS) – add a certain column class to your currency column (ex. currencycolumn) and add the following styling to your stylesheets:
.currencycolumn > div:before {
content: '$ ';
}
There’s also ways to combine these two tricks to show various currencies via styling but it’s quite complex.
For input fields it’s not as easy in Mendix – it’s possible by adding a container around the input field and a text element in the container. Then use position absolute & relative to position the text element at the front of the input field, and use text-indent to move the contents of the input field to clear the space used for the currency sign.
Hope this more-or-less paints a picture of how you could implement this.