Edit non-standard form fields?

I know that we can do some modifications to some form fields like the subject, etc…but what about any other field? My need is simple, adding a dollar sign ($) in front of what gets mailed for a specific field. I tried the following…but it didn’t work. I based in on what is used to modify the subject.

// add dollar sign to field
	if ( is_page( 'quote-request' ) ) {
	$budget = '$'. urldecode($new_post['3_1']);
	return $budget;
	}

Is there a specific function something like this has to be passed through? The end result would be when someone puts something like…350 into the field, it is emailed to read “$350”. As a feature request, I guess would be that we could designate a field in the builder as numbers only and/or currency. Both would be a simple designation to add and would allow us to build more advanced forms without the need to install yet another plugin.