Thanks for the question. I believe you are running into this because we don't have currency support in Tableau Prep at the moment. Here is workaround that you could try:
1. Remove the '$' in front using by creating a calculated field using the following calc: REPLACE([Currency], "$", "")
2. Change the data type for the calculated field
Thanks for your response. Is this something that will be added in a future version?
Yes this is something we are working on. We are looking into allowing that data type conversion and not losing the currency information.