I know for opportunities Salesforce doesn't have the actual curreny in the field, just the number. There is a different field for currency. So Marketo actually displays the data correctly. If everything is in Dollars, you could just manually add the Dollar sign in your emails. If you work with multiple currencies (like we do) you could use the currency field, but that displays like EUR or USD. So {{company.Currency}} {{company.Amount}} would display: USD 20000.
If you need numver formatting, create a new custom field in Marketo (type currency). And populate it with the available source field you have now.