I think its like 16K xp per brew of correct affinity and like 12K xp brew for different affinity....there about

but no other negative, just less xp per brew than if you use it on the correct affinity

Yes, you just get less EXP if you use another affinity's brew on a champion. If you wanna be more efficient with brews/silver spent, just use the same affinity.  

Oke thanks