If you bought a bike, for $450 and had it serviced twice for $150 each time and got 10000 miles out of it that works out to: $750/10000mi = 7.5 cents per mile. That sounds like you are getting pretty good use from your bike. Keep riding it. Sometimes slightly more expensive moving parts like derailleurs and bottom brackets last for more miles of riding before wearing out, but if you got 10000 miles out of the original derailleurs, get the same derailleurs again when those wear out.