I get the feeling this equation would be a bit more useful if Passion was a more prominent factor in it. As it is, it's hidden in the Value variable.

It could be a bit misleading. For instance, I automatically was thinking about monetary value and usefulness and such, while completely forgetting to include the Passion factor. If the Passion isn't weighted heavily in the equation, motivation might turn out to be low regardless of how high the "Value" (besides passion) might be.