How can I purchase the Project Web App? How can I deploy Project Web App in a SharePoint Farm?
To answer these and set expectations: Microsoft Project Web App also referred to as PWA is a component of Microsoft Project Server 2010. So to answer the questions above, PWA comes when you purchase Project Server 2010 and is not sold separately nor free of charges like the Office 2010 Web App; and to deploy PWA you need to deploy Project Server first.
More Info: Deployment for Project Server 2010