I have a friend that bought two of the inflatable kayak models in the $100 price range for playing in lakes. They are made of fairly light PVC and soon had several pin hole leaks, even with careful handeling.
After a year of light use he probaly spent another $100 bucks on patch kits and lost time fixing them. I wouldn't want to bet my life on one. Even in a lake you might find yourself in a wind storm situation with some big waves.

If you shop around on the web for used rafting gear you can usually find something like an Aire 'Lynx' or 'Tomcat' for under $500. These are real whitewater boats that will give years of service plus the confidence that it would ride out a storm or survive getting blown into a tree root.