The only rule is players should agree on the use of special pilots and which ones (AGoAC rulebook, page 48).
There’s a rough guideline of using similar SPA costs (later on the cards), but as you say that can vary a lot.
Battle Value/Point Value do not cover them, they remain entirely optional and at player’s discretion for when to use them. Essentially they've been added for fun, without a great deal of balancing.