Free: Contests & Raffles.
Cheapest one out there is the Marsh Rat I believe. Only thing I don't like about it is the new plastic model deforms pretty bad when layed on various items or when it gets hot.
I don't think you'd want to build one while the season is going on due to time and weather issues (for the resin), but if you end up waiting until post-season spring/summer you could always build your own. Fun project and it's reasonably forgiving. I built the SN over the tail end of the summer and am quite happy with it, especially considering how very not handy I am. Takes a bit to get used to paddling around though - I'm looking for a 2hp for mine. http://www.hybridduckboat.com/I've got a friend with an Aquapod and he loves it, so I don't think you could go wrong there if you aren't wanting to build.