Made in china bars for 100 bucks tend to be cheaply made on the inside. The 600 dollar ones will have better bearings, etc... Wether or not the price is worth while basically depends on how much weight your handling. A 600 dollar bar will handle more weight, and will feel different when loaded. If you're on a budget, and aren't elite-total strong, buy the cheap bar. Oly dumbelle handels are nice, but they can get pretty long. If you're just starting your home gym, and are on a budget, I'd consider buying a bit of 1 inch cold rolled round bar, cutting it to length, and popping a couple of plastic "olympic adapter" sleaves on it. Or just buy 2 inch round bar, and you'll have fat bar dumbelles. Than save your pennys to buy a "proper" set of dumbelles down the road for your finer accessory work where 20 inch long dumbelles aren't so handy.
You usually get what you pay for, but don't let that stop you from deal hunting. Some of the equipment on the internet is marked up alot more than it should be. The thickness of the steal, quality of the welds, over all design, etc... of a rack is more important than the decals, so have a look at whatever you can in person before you buy. If you want things on the cheap, be realistic about your strength levels, and your goals. A good rack will last forever, no matter how strong you get, but it takes a long time to get strong enough to break even a cheap rack. Craigslist, and kijiji are your friends though. Check that first.