What's the distance from the camera to the plate read?
I think you could get away with a fairly low cost camera, but in order to reliably get plates, you're likely only going to get plates. You're probably going to need to set shutter speed to 1/1000s or faster, which is going to darken the scene, even during the day, but you should be able to read it at night.
See our Low Cost License Plate Capture Shootout for examples. The Hikvision 2032 did fairly well here because its IR pattern is more even than some of the others. But based on our 4MP tests recently, both Hikvison and Dahua are pretty even now (plus there's an increase in PPF with the new 4MP sensors).
Also, since a lot of the lower cost cameras are fixed lens you'll need to look for longer focal length models or be very close to the area you want to capture the plate in. Longer lenses are often an option in Hikvision/Dahua bullets (and others), with 2.8 or 4mm as default. If you can get an 8 or 12mm you can get more distance.