Well I think that $5 would be a great price for a portable version and hopefully make the development time worth it. Few other developers I really like do that. Desktop versions free, mobile costs. Either way hopefully you decide to, would enjoy doing this to and from work on public transportation.
Edit: Also, if you could port it as is, with the ability to zoom in and out that would be awesome. Though I would use this on my phone, on my tablet as is would be fine. Check out SunVox if you get a chance. Has one of the best interfaces for this sort of thing on mobile.