Arbitrage is defined as the simultaneous buying and selling of securities, currency, or commodities in different markets or in derivative forms in order to take advantage of differing prices for the same asset.
Case in point: The CUI VSK-S10-12UA-T. DigiKey wants $31.79 for one. And they're backordered anyway. I use them to build Hydras.
But the CUI VSK-S10-12UA is only $20.13.
Check the specs - it's just about exactly the same thing. The only difference is that it's through-hole PCB mount instead of chassis mount. In fact, it wouldn't surprise me greatly if the chassis mount variant didn't simply consist of a plastic chassis mount container holding one of the board mount modules!
Well, how much could it possibly cost to make a board with four holes in the corners?
The answer is $3.40. Well, that's just for the board, and assuming a medium-run order ($1/in²). For the normal order from OSH Park, multiply that by 5/3. Add a power LED and resistor and a couple of screw terminals and it'll be a dollar or so more than that. You could even add the recommended external filter caps and TVS while you're at it and still be certainly spending less than eleven dollars.
This doesn't even break down much over quantities - The chassis mount variant is still almost $10 more when you buy 500 of them at a time - more than $4000!
Now, I'm not sure my little store would be able to beat DigiKey and CUI at their own game - I'm not sure there's quite enough profit or a big enough market for me to sell ersatz VSK-S10-12UA-Ts done this way. But I'm sure going to do it to build Hydras from now on...