If it's for the life of the blog, I don't think anyone would have a problem paying a one-time fee. I'd also be interested in hearing if you can import posts back into Tumblr as well. (This would be useful for people who want to clone a secondary blog into a primary)
EDIT: Based on the signup page[1], there are a handful of steps that likely represent what you'll do when you backup the blog. They seem to indicate more of a one-and-done process.