$20-$30 is not unreasonable for the amount of content they are putting out in this DLC. Also, with full retail games up next to games below $10 on the eShop, this particular DLC will not look out of place or offensive at a possible price point between $20-$30 due to the amount of content that's being offered.
That being said, I like the suggestion that it should be offered as packs of levels or worlds and as one whole lump sum at better value. But I suspect it will be released only as a whole as this is suppose to be a unique adventure for Luigi and not just a pack of time trial or coin rush levels with no context between the two (or as much context as any mario game level has).