Michael,
Another relatively inexpensive hood that works is the folding barndoor style SOOPD made for the Summitar. I picked one up for $19 within the past year. I got the version with the set screw. You also may have to remove the inner band that the set screw tightens against from the hood to make it fit. That does mean you are tightening the set screw against the frame of the lens or filter if one is mounted which could nick the finish if that is a concern. It is uglier than the XOONS, but the folding sides serve as a lens cap and I can use it on my 50 collapsible Summicron.
I would say price of the XOONS is based on rarity and some think it is a really nice looking hood. Plus, to minimize flare the Summarit really needs a hood. I have not seen near the number of XOONS available as there are Summarits. I do have a XOONS, but it stays at home sometimes.