Got an email from onlinemetals.com when they were doing their Black Monday sale last week. I poked around a bit and saw that they have value packs, which I never noticed before. If you select the 'View All Types' option for a particular metal (brass, steel, aluminum, etc), the very last two options are Value Packs. The Protobox items there are grab bags - about a buck a pound for cold roll, two bucks a pound for aluminum.
I picked up a couple of ten-pound protoboxes (cold roll, aluminum), half-expecting them to be a bunch of thin flats. They arrived yesterday and actually contain a decent selection of material: some flats, some angle iron, some round, some tube. Plenty of good, thick, bulky pieces which are usually kinda pricey to obtain on their own.
Not junkyard cheap, sure. And of course the shipping still costs ya. But a couple of these could get you started for, what, fifty bucks? A hundred if you include brass and some drill rod? Something to keep in mind for those starting out with only the stock at the local hardware store to draw from.