Google XML Sitemap Creation - Weekly

Hi, We need a freelancer to create a Google Sitemap in XML format once a week and send it to us so we can add it to our sitemaps account at Google.com. Our site is very large, as it is a phentermine information portal with a few main pages and a forum containing 2800 users and 17,000 posts. Please let us know if you can do this job weekly and how much you will charge per month to do so. Thanks!!