Hello Joe,
there is an un orthadox way to satisfy your need.
You should enable to form to enter text data and make sure you validate the data.
Validation should be like : Must allow Number (0-9) and $ and Period .
Lenght of data - $ XXXX.XX or $ XXXXX.XX
Later using java script: you can truncate the $ value in form logic and append the $ for the Result.
Hope this helps you to a little extent.