For 1 champion and 1 homeworld, it seems about right. Having a champion makes satellite worlds nigh impossible to defend, but you get the champion itself, 98 minion starships (while not as good as full starships, they add a lot of fast-moving punch to your fleet), 3 mod forts, significant economic benefits, and 4500 k (a bit more with duplicate nebulae).
For the second champion, the response doubles, but all you get out of it is one more champion ship. For further champions, the response increases just as much, but the benefit is still at just one more champ.
For additional homeworlds, the benefit increases, but not the cost.
I recommend something like this:
Make nebula ship caps and mod fort caps not scale with # of Homeworlds.
Make additional champs only add about a third as much as the first to anti-champ response.