We were advised by Mendix support to use Text widget to display decimal attribute with a $ sign followed by a parameter (${1}) in order to display decimal field as currency field on UI. That is workable solution until you come across a negative value. For e.g., -1000.00 would appear as $-1000.00, which is absurd. Upon asking further for a better solution, I was advised to ask in forums. I know there are some widgets available, but I believe this is a very common requirement in an application and Mendix should incorporate this within the modeler. Also, I know I can write a microflow for a calculated field to address this issue, but again not sure how much oof performance hit is involved if we keep on adding calculated fields to a mendix application. Anyway, let me know if there is a possible workaround for this problem.
Other sollution would be to use the token replacer from the model reflection. There you can set a display pattern with different pattern for negative values.