Use a piece of chalk and it runs the risk of not looking official enough and the police will just get accused of making it up. A proper piece of kit makes it all correct, calibrated and consistent so people might pay more attention. Remember that bad drivers will look for excuses to justify their driving and "it was just some copper with a piece of chalk" would provide just that excuse.
If the fuzz really do decide they need it then it should be funded out of driving penalty fines or something though. Maybe even general taxation if it's a genuinely useful policing tool. What's next, crowdfunded tasers?