I tested this regex in php flavour. This will not match $500 since you want more than $500. This will detect either exact 2 decimal places or whole amount. This also handles case scenario where 600, 700 etc were ignored because we were ignoring 500.
\$([5-9]+\d[1-9]|[6-9]+\d{2}|[1-9]+\d{3,})(\.\d{2})?