4k miles is about right for a soft compound tire on a cb750...I get about 4-7k miles out of what ever tire, ride hard, and usually leave a back tire on quite aways past what is prudent ...because yeah they are expensive. I think I got the most miles out of a Metzeler. Dunlop seem to wear out the fastest but are great in the process. One thing if you don't mount your own tires...if you have a decent bike shop in the area, they will usually mount and balance for cheap/free if you just pay a little more and buy the tire from them...