I suggested a 2 characters for $15 before, and I still think that would work.
1 Marvel and 1 Capcom character per DLC "pack". Throw in a stage and some alternate costumes as well.
It would also be wise to include one "popular" character with one less popular character. The less popular characters wouldn't sell as DLC, so you use the popular character to increase the attractiveness of the package. I don't know how well Nebula sells at $7.49, but Nebula and Wesker for $14.99? $$$$$$$$$$$$$$$$$$$$$$$$$
1 Marvel and 1 Capcom character per DLC "pack". Throw in a stage and some alternate costumes as well.
It would also be wise to include one "popular" character with one less popular character. The less popular characters wouldn't sell as DLC, so you use the popular character to increase the attractiveness of the package. I don't know how well Nebula sells at $7.49, but Nebula and Wesker for $14.99? $$$$$$$$$$$$$$$$$$$$$$$$$