you have to build every monokuma type. once you build warlockuma (the final monokuma), you get 200 coins.
it's also how you get the school mode endings. so don't use your trip tickets if you messed up and couldn't get a monokuma built. just use the free time to max out the report cards. you should be able to get about 3 endings per playthrough. the only endings i have yet to see are for sakura, celeste, mukuro (as junko), kyoko, and hina.
There's just barely enough free time to get four endings per playthrough if you're lucky and make the right dialogue choices for everyone.