RDLC textbox properties to currency

This should work, (not tested):

In the Text Box Properties dialog, go to the Number tab. Under Category, select Currency. In the Locale dropdown, select English (United States) to ensure the currency symbol is set to USD ($). Choose the desired format from the Sample section to see how it will look.

The XML should look like this for 2 decimal places :

<Textbox Name="textbox1">
  <Value>=Fields!Amount.Value</Value>
  <Style>
    <Format>$#,##0.00</Format>
  </Style>
</Textbox>