For more complex Privacy Policies, the cost could be anywhere from $1,000 to $2,500 or more.
You would also benefit from reading the Privacy Policies of apps or websites similar to your own to see what information and clauses they include that you may not have thought about.
This simple example is comparable to most small ecommerce websites that process orders and collect money. These Privacy Policies tend to be shorter and less complex, but it is still a good idea to have an attorney involved so you can be sure your website is compliant with all relevant laws when handling customer's credit card and account information.
For the $500 cost it's worth having an attorney do it right rather than doing it yourself and risk costly legal issues in the future.
For example, a website that sells products, offers training services, and has user-generated content will need to be sure it addresses the needs of each of those categories, thus increasing the price to perhaps up to three times what it would cost to address a single category.