I stumbled upon your question because I was looking for the exact same thing. I finally found something on Amazon, apparently "reverse hole" should do it for searching. This particular item was exactly what I was looking for as I wanted to secure these wooden "box" cabinets I have to each other (as they are currently only connected ...
I haven't priced flat steel stock, but you could make something triangular with no welding skill. You'd just need to drill 2 holes in the part that goes against the wall and an additional hole at the bottom of the hypotenuse. I'm still not sure you'll come in under 4 bucks, though.
No, you cannot make an ornate painted metal wall bracket for "cheap", ie for considerably less than $4, using any process that I would call "DIY" in the context of hanging a plant. If the question was "I have a metal shop, what's the cheapest approach to reproducing this piece", perhaps there would be a much-less-than-$4 one ...
A block of aluminium for each side. The vertical hole may need tapping, but the horizontal one could be drilled straight through, as the bolt going that way would need to thread into the drill (judging by the diagram).
It may even be possible to drill straight through both, depending how the vertical thread needs to be fixed.
Get some 3/8-16 studding, aka threaded bar, and bend it. Your diagram suggests you don't need a sharp bend. A bench vice and hammer would suffice to make the bend in something as light as 3/8 threaded.
There is such a thing as a hinged threaded standoff, that is basically a non-rigid version of what you're looking for. (No endorsement implied, in fact the linked company doesn't even carry them in the size you need.) They are available with every combination of male & female threaded ends.
Do think you will have to make up something.
Street elbows are for plumbing and thread size/type does not match.
I would use a long type of nut(as shown in @JPH1618's answer), and cut it in half at 45 angle. Weld the two half's together to make a 90 degree angle.
Could also weld plain nuts to an angle bracket.
Pipe thread and screw thread are different even if you could find the right size elbow. What you might have to do is take a threaded coupler or two and have them welded together.
I've never seen a threaded 90 like that, but it might exist for some specialized purpose.