Here's a question. Why the assumption that a set of mini-figures would be cheaper than a Mega Muppet or a Playset? Maybe a simpler line like Simpson's, with the less complicated sculpts and simpler paint jobs can carry that, but I doubt it for Muppets.
If we were to do a set of, say, six mini PVC figurines, you would want them to be detailed right? Far better than anything that had come before...textures, good paints and all that. So, taking that into account, and comparing it to other lower volume, higher detailed PVC sets like the ones from DC Direct or Dark Horse or the like, you might be looking at a 24.99-29.99 retail price. That factors in complicated sculpts that require more than a 2-part mold to make, extra parts and details and multiple spray masks for detailed paint jobs.
Not sure what the Simpson's line in the smalls is running these days, but the most recent PVC sets I have bought were Danger Girl and Hellboy and Justice Society. These had a real high level of quality. Granted they were specialty market only but you might have to go there with Muppets because of the fact that there is no guarantee TRU would take them, as the PVC set is a difficult sell in a lot of cases, particularly to them with no tie-in.
If we WERE to do a line of smaller figures, on a personal note I would expect them to be new sculpts, not rehashes of the existing figures. That works on a MEGA line, but not on a PVC line. And I also, and this is just my opinion here, would dig on themed packs like the current Simpson's. Plus with a multi-pack there is the theory that you could go WAY more obscure with some of the characters. By packing a Kermit or Gonzo in a pack of six figurines you can get away with putting a Mahna Mahna in there because from a marketing perspective you will have a character or two in there that the non hardcore will recognize and the hardcore will really want to see made, but who would probably not be able to carry his own figure. You know what else would be cool? Arch display stands. Mount on the wall or stackable...that would be cool to me. I imagine a wall full of Muppets, stretching as far as the eye can see...in arches, now THAT might be nifty.
Contrary to popular belief, bigger doesn't always mean better!