Replacing a complete car battery, especially one that is used extensively as a hybrid battery, is not like replacing the battery in your TV. It is a very difficult, difficult procedure, and, if approached incorrectly, there is a real risk of electrocution and serious injury. Unless you’re a seasoned DIY pro, any money you save isn’t worth the harm.
Toyota doesn’t provide official documentation for Prius battery housing replacements, probably because they don’t want to be held liable for people who try it themselves and get hurt. So, if you want to do this, you should see a DIY book or video, which may be outdated or wrong. Also, if something goes wrong and is replaced by an authorized garage, they usually offer compensation and free replacements. If you do it alone, you won’t achieve success if you are confused.