Some people saying FREE!!! could use an economics lesson. There's no reason for them to make DLC unless they can pay their staff to do so while also making a worthwhile profit.
As it stands, there's 27 playable characters (28 for PS3 users), which comes to about $2 per character for a $60 game (that's not factoring in stages, etc). So anything over $3 per DLC character, I'd probably scoff at. A "$3 per character, $10 for pack of 4" model I think is the upper end of what I'd accept.
Stages would be a hard sell individually. They'd have to have some really cool elements (stage fatalities) for me to bite. $1 per stage (which sounds too low) is about what I'd pay. Packaged in with other content, that would make it easier for me to accept though.
The best package would probably be inclusive packages that have DLC characters, alts, stages, fatalities, etc included. That way if you can't justify spending $$$ on ONE costume for a character you don't even main, at least you'd still be getting something in the package that you consider valuable.
Either that or a package for additional content (fatalities, alts, stages) and a separate package of characters would be effective, IMO.