Adding a dollar sign in front of a number in a random text/numeral string

I am helping a client that wants to make sure that a dollar sign is in the appropriate place within a varchar field. I have 90% done however there is one instance that I didn't program for. Below are the a few possible scenarios that I have programed for, as well as the attached code.

50.00
only 50.00
only 50.00 for 4 of them

Now based on the attached code, I was successful in adding the dollar sign in front of the 50.00 so the output will look like this:

$50.00
only $50.00
only $50.00 for 4 of them

The problem is that when the user types in:
4 for only 50.00
The attached code outputs
$4 for only 50.00

Basically I need to modify or rewrite the script to account for that issue. My current script grabs the first number in the string, then adds a dollar sign to it which is fine until a number that deals with quantity is added first. I realize that it would be easier to add a price field and a price description field, but I am not able to convince the client to go that route. Thanks for your help. Oh, the client doesn't mind if we add the dollar sign in front of the first instance of two numbers... if that helps. Thanks!

$price = "4 for only 50.00";;
        $price = str_replace("$","",$price);
        $just_numbers = ereg_replace("[^0-9]", "", $price);
        $first_number = $just_numbers[0];
        $new_number = "$]" . $first_number;
	$pattern = "/$first_number/";
	$replacement = "$new_number";
        $price = preg_replace($pattern, $replacement, $price, 1);
	$price = preg_replace('/$]/', '/$/', $price, 1);
	$price = str_replace("]","",$price);
        echo "$price";

Select allOpen in new window