Is the $30 a one-time, flat fee?
Because Mint is not a hosted service, its license is comparable to that of traditional desktop software. The $30 dollars is a one-time, flat fee per installation for the current version of Mint.
Updates (eg. 1.0 to 1.2) are free for licensed users. Updates are released as new bugs are uncovered and squashed or minor features added.
Because of the extensibility provided by the Pepper plugin architecture, full version upgrades (eg. 1.0 to 2.0) will be infrequent. Upgrades are available at a reduced price for licensed users with a short grandfather clause for recent license purchasers.