Depending on the make and model, materials, and features, you can expect to pay anywhere from 30 bucks to several hundred dollars or more for a neck brace. High-end collars are made of expensive materials such as carbon fibers which make them strong and lightweight. These are usually intended for extreme sports such as MX.
However, you don’t need to spend a fortune to get a good amount of protection. There are quite a few models that come with a solid design that should keep you protected in case of a crash.