As good as it looks, 5 levels is fairly little for USD$5. If you made it, say, 7-10 levels it would be more reasonable, but you never know.
I'd also throw in a couple of multiplayer conversions into the mix, since, like I said, it's not a whole bunch (and, if you only briefly edited the single player levels to multiplayer, it wouldn't take much work, either).
And, especially if the levels are short ("a few minutes long"), then I'd
definetely consider putting more levels into it. On the other hand, if you utilise numerous custom scripts, cutscenes, and story techniques, it makes more sense.

Evil has a new name. Demo out now!