Read the Price Tag

Prices copied from a website arrive as text with junk around them: spaces, tabs, line breaks and dollar signs, at either end, in any mix.

Task: write price_value(tag) that returns the price as a float.

• Only the ends are messy. The number in the middle is always a normal number like 19.99 or 5.
• There may be several $ signs, and spaces can sit between them and the number.