It's not about monthly costs. It's about up-front costs. These systems can be millions, maybe tens of millions to start, and are typically a huge, multi-year contract to a third party company that manages the system. Plus hiring human staff, maintenance and installation of a huge amout of it systems, training, tying in the car registration databases... On top of the fact that they're politically unpopular. Most people don't want to be tracked everywhere they go, even if the trade off is slower drivers on the road. There's a lot of headwinds before monthly costs are ever even a consideration.

Speed limits aren't an "everyone gets to be right/wrong" thing at all. It's numerous new things added as band-aid and one-item "fixes" to specific situations. It's also why the US tax code is such a mess. No one cares to overhaul it, and it's just a huge jumble of singular tweaks. It's like when you let a group of people take 10 years to write a story, and every week 5 people get to add or edit 1 sentence. Will you get a harmonized story with coherent plot? Likely not.

Radar guns don't report the speeding to anyone. A radar gun sitting on the side of a county road will write a ticket... how? Magic? It's a tool. A tape measure doesn't measure your height for you, you have to read the number and know the context. Before radar guns cops had to base speeding on a guess, or their own speedometer, and had to testify in court to a more subjective standard.

We don't control speeding because we mostly have passive systems that fine people for violations. And anyone determined enough will speed anyway, fines be damned. Many people speed as a flex and pay fines like a status symbol. You can't engineer 200km or 200 miles of highway to keep people at 65mph or below. Engineering solutions are limited to pedestrian areas or slow speed areas. The systems we use to fine people have the added societal costs in that they take taxes to set up, and are surveillance state tools.