If you spent $300 to balance just the crank & rods, then you payed alot and the job isn't finsihed yet. If you payed $300 for balancing the whole rotating assembly,(crank, rods, pistons, balancer & flywheel ect) then you did good. (Mallory metal gets expensive, so they obviously didn't need to use much of it) Balancing should always be done for any serious rebuild or performance engine. And high RPMs isn't the sole reason you should balance your engines assembly.
Vibration can occure at any RPM and you won't always feel it. These imbalances can cause cracks & fractures in crankshafts and also lead to premature bearing wear or failure!! (Example: One reason gear drives are NOT such a great idea for street engines is the fact that they link the crank to the cam through gear to gear contact. This contact can transmit any crankshaft vibration to the camshaft and cause big problems) When an assembly is balanced, it's not perfectly balanced for ALL RPMs.
It's balancing coverage is wide enough to effectively dampen most harmfull vibrations that occure throughout the engines rev range. When the assembly is balanced, the shop merely guesses at the amount of oil the crank will have on it, in it's oil passages etc. Same goes with the pistons and oil rings. Also the oil flying around in a running engine will be be different from momement to momement and this will change the actual measured component weights as this oil clings to them. Then there are primary and secondary imbalances to address, so any shop that claims they can "perfectly" balance an assembly is full of it! Maybe they can perfectly match weight the rods & pistons to each other, but they'll never be able to guestimate what is actually happening in a running engine that's under various loads at any given momement. In short, you did the right thing by balancing your assembly and your engine will last longer and produce a bit more power.. Good luck & drive FAST!!