The definition of a rule is  an official regulation, code of regulations or set practice. An example of a rule is that a red light means stop. An example of a rule is an employer demanding their employees arrive at 8am.
New injectors cost anywhere from $80-$120 apiece, making their replacement a somewhat pricey endeavor. There are three types of injector leaks-it can leak fuel into the manifold from the nozzle, it can leak fuel into the engine compartment from the fuel rail, and it can leak air (vacuum leak) from the manifold.
It depends. If you have a twin turbo BMW (135i, 335i, 535i) with the N54 engine, the most common problem is the High Pressure Fuel Pump (HPFP) which has an extended warranty that covers the pump to 100K miles, and by then they will have hopefully solved whatever design flaw is causing them to fail like crazy.