import re
s = 'Your bill of USD 17.99 is due on 09-01-2002'
match = re.search( r"bill.*?([\d]+\.[\d]{2})", s.lower() )
if match:
print match.group(1)
aW1wb3J0IHJlCgpzID0gJ1lvdXIgYmlsbCBvZiBVU0QgMTcuOTkgaXMgZHVlIG9uIDA5LTAxLTIwMDInCgptYXRjaCA9IHJlLnNlYXJjaCggciJiaWxsLio/KFtcZF0rXC5bXGRdezJ9KSIsIHMubG93ZXIoKSApCmlmIG1hdGNoOiAKICAgIHByaW50IG1hdGNoLmdyb3VwKDEp