The easiest way I found to get the points needed to buy the extra characters is as such-
Start up a training fight, any characters will do. When youre done sparring (or not, you don't even have to move the character any) Dont pause it, just let it run- turn the tv off, leave the console on- it matters not. Because you earn roughly 10 points every minute you spend "sparring". In two hours I earned over 2k points and used it to buy roughly two characters.