Tag Users and Taxonomy terms in body field of a post
It will be interesting logic to figure out whether $art is a user or the start of the $art and entertainment taxonomy term, but, regardless, what you are most probably looking for is an input filter that can recognize a pattern in the text and then both create a link to either the user or taxonomy term as well as actually tagging the node with that taxonomy term if it is one.

A start might be to take a peek at the Hashtags Module and change its use of # to $ for the taxonomy portion and then extend it to include users.