Well, you've got to consider who you are catering to when it comes to selling NP$ hosting. This applies to selling hosting for dollars as well.
It's safe to say that cheaper prices attract cheaper people. Right now, your cheapest plan comes out to $3.40/month or 200 posts a month which is cheap in the hosting industry for such a small package. IMHO this is a fair price given the fact that it doesn't necessarily cost the person paying for it a thing. 200 posts a month here at a thriving forum is nothing.
Getting back to the point though, listening to your customers and especially potential customers can only go so far. Ultimately, they don't have your interests at heart. They want the best deal they can get for the cheapest price and they could care less how much you get out of it. That's where you have to know your costs and make the right decisions which is all part of running a business.
NP$ hosting is a tough arena because generally these are the people who like free hosting and want it all for virtually nothing. Don't get me wrong, I'm not saying all NP$ clients are bad, it's just the mentality behind it. A NP$, unlike money, can be earned with relatively little effort. Afterall, if you signup to post at a forum it's probably because you enjoy posting.