As far as motor life, and rebuilds, I rebuild mine about every three years or so. I have never HAD to do a rebuild because of melt-down or any other reason. But, when I tear into it after three hard summers of riding, I find its running at about the upper limit of acceptable piston to cylinder clearances. Please don't take that statement as a rule of thumb for your own use. Depending on how many hours you ride, how hard you ride, how heavily modified yours is (mine is not even remotely stock), how well maintained, etc... it all makes a significant difference on motor life.
As far as money, its not that expensive to do a rebuild provided you do your own wrenching. My last rebuild ran me about $200 (All new parts and gaskets with boring incl). Add up a rebuild for a Honda 250R with all Honda parts, and I think you might be surprised. I haven't checked since I sold my 250R, but, it was over $150. So, really, rebuilding is not a major worry. Doing modifications (pipes and silencers, carbs ...) in double is sometimes a bummer for your pocket book, but, it's all worth it when you can pull up to a 500 and blow it away in a drag race. No offense to the Quadzilla riders out there, I really envy your low-end grunt that my Banshee will never have. You have to give something in order to take hi-rev horsepower. For me, that's an easy decision. This sorta leads into the next question ...