Princess Alison: From the brains page;

For example, if you want to buy one-year Brain Knight, you must have at least 500 Brains. When you have enough Brains to buy the particular membership, a new form will appear on the Profile page of the user you want to buy a membership (yours or anybody else's) and you will be able to use this way of payment.


So you would need at least 500 brains before you can get a membership.