$191 US:
https://www.ebay.com/itm/Cycliq-Fly1...UAAOSwOYFaXXfJ
I have heard people say the rear-facing cameras are not as helpful, so rather than get two cheap front and rear cameras, I would personally just get one good front-facing one. The battery life on these is 4-5 hours in real life conditions. Any of the other hi-res cameras that will pick up a plate have much shorter battery life (80-90 minutes?). If so that might work, I have seen Go Pros at Walmart, with bike mounts.
With my Fly 12 CE, the resolution is not great, but if there's an incident, the resolution (and sound) are more than enough to document it.
Another cheaper option might be to mount a still camera, and have it ready to take still shots. But if you want to have something that runs continuously for many hours, I think Cycliq is still pretty much the only game in town. Resolution is not as good as Go Pro, but i think the battery life makes up for it.