3-side punched steel channel (strut), image in the post - can't find any equivalents
That stuff is called Unistrut.   Comes in a variety of configurations.   Keep in mind the normal way to attach to it is put special clips with springs that latch into those tucked-in grooves.  They're not just for strength.  

When you see stupid pricing on the Internet, that's the "I don't want to sell it" price, or to be more precise, the "I don't want to ship it" price.  They are trolling for noobs who don't know or care what normal prices are, and will pay whatever.  Anything like that - heavy and cheap - should be sourced locally.  If that means you need to skill up on procurement, then definitely do that.  Grainger, lumber yards, local metal supply, electrical supply houses, cast a wide net.