Well, a hundred bucks is cheap for an Iwata! But we're not looking for name brands here!Unlike HL vs Tamiya debates. This my friend is black and white.A $400 dollar airbrush and a $20 dollar airbrush will produce the exact same results in the right hands. Now, I use "right hands" loosely. No airbrush will make you a miracle worker just by depressing the button.
Various airbrushes from various companies will be constructed with various materials. I've tested and own quite a few.

The ones you can get at harbor freight are aluminum....cheap and light. But, I've owned it for 6-7 years and it still looks brand new.
Master brand is made of brass and maybe some steel components.....like Iwata and most of Paasche's brands. Badger too has come up, but some of their brushes utilize plastic...mostly in the handle, but their lower brandisalmost all plastic.

But, if your willing to spring for the Paasche Millennium, go for it. I'm just telling you from experience. I can paint you something with my sons cheap Badger 250 and my Iwata 4208 and you'll have a difficult time telling the difference.