Roger Jones has a great site on how to build your own Boat Monitoring System complete with step by step instructions on how to build the hardware and a shareware application that he wrote for you to use as well. In total you need about $250 worth of gear + an old laptop which I’m guess you can acquire for under $100 on Ebay. He claims you can do this for under $400. The system monitors all your basics systems and can take actions like turning off lights. It also plugs into your NEMA network and can display navigational informational as well, is your boat moving when it should be at the dock?
I’m not sure that this is really any cheaper than the off the self boat monitoring systems, but the shear coolness of building things yourself, and thus being being able to troubleshoot and fix them is awesome. I recently did a little soldering to convert some headphones and that was fun, I would love to tackle a project like this someday!