While you're developing your site, you don't have to pay a dime until you are ready to take it live by publishing to a load-balanced, fully monitored, and worry-free live instance (i.e. group of two or more servers). In the meantime, you can check your work in the preview environment, which gets updated with every save and fully replicates the live instance environment.
As a beta user you'll pay nothing until your site is live and have immediate lifetime access to all new features released during the beta. When you go-live during the beta, you'll get the benefit of a reduced monthly fee for all the same features you've been using.