SQL Query - How do I convert a text field containing dollar sign to decimal only (strip out dollar sign)

SQL Query - How do I convert a text field containing dollar sign to
decimal only (strip out dollar sign)

I am running a query on a MySQl database and I have a field of text
characters that contains a dollar amount with a dollar sign and a number
(ie $1,345.89) I need to do a check for not NULL (some of the text fields
are NULL) and that the value is greater than 0. How can I convert this
text field to a decimal value so that I can verify that it is greater than
0. I do not want to alter the original text in the database.
Thanks