Whatchathink...Mike is exactly right. Liability has nothing to do with fee level.
Even a simple services authorization form would give you the contractural protection you need. It doesn't have to be some long and laborious contract. My form, including where I outline the general scope of work, the expected fees and the client contact information is only two pages, but it has MY contract terms and general conditions. About 90 percent of my work is done this way, with fee levels ranging from $1000 to many times that.
Under the California law, if you used the waiver, the client could then sue you for malpractice if something went wrong, and you would have no contractual defenses...for instance, indemnification for attorney's fees or limitation of liability. What if you do the work and then don't get paid? You have no contractual basis for collection.
I would use a simplified contract rather than a contract waiver unless such waiver provided clear indemnification and a waiver of legal action, including arbitration and mediation, in the event of an issue.