My colo just put the price of power up, from £17 per amp per month, to £30. I use 4 amps, so that's an increase from £68 to £120.
At a colocation, you pay for three resources - space, amps and bandwidth. The bandwidth is usually 100 mbit, but how much you need, depends on how much you need, if you see what I mean. It's hard to make economies. But space and power ... you can. 1U cases are 1.75 inches high, that's the standard for 19 inch racks.
When I got home from my bike ride, I put a powermeter on a few servers. My conventional 1U servers pull 500 milliamps (half an amp) at 240 volts. But the pi pulls 500 milliamps at 5 volts, so when I put the power meter on the transformer that supplies it, I wasn't at all surprised to see that it pulls 10 milliamps at 240 volts (which it converts to 500 milliamps at 5 volts).
So here's what I'm thinking. Take a 1U case, and a 1U power supply. Line the case with tape, to guard against pies getting short circuits if they touch the case. Attach the pies using tiny magnets. Bodge the power supply so that it will power on even though it can't see a motherboard (that's an easy fix, you connect one wire to short a couple of the pins on the output). Use the 5 volt output to drive, say, eight pies. Run the power via a relay board that allows me to switch on or off any of the eight pies (being able to remotely cycle the power is very useful), and that board runs off 12 volts, also available from the power supply. Put in an 8-way ethernet switch, also powered by 12 volts. Put in a few fans to keep the air moving inside, also running off 12 volts. The total power draw, with all pies on, will be 80 milliamps for the pies, and a bit more for the ethernet switch and fans. One ethernet lead comes out of the box, and one power cable. So that'll probably be about a tenth of an amp or so for the whole thing.
Actually, I can get at least 16 pies in one 1U case. Pulling a fifth of an amp. Compare with a normal 1U server pulling half an amp.
Well, it's not quite as simple as that - you can't connect hard drives to the pies. So what I'd do, is have six 3TB hard drives in a 1U case (which would be about 0.5 amps), then six more in another (as backup), and six more in a third, but powered off, so not costing any power draw. And the pies would access them across the network, using nfs or Samba. Which means I wind up using about 1.2 amps instead of four. And the space taken up is less also.
So, first thing is, to make one of these things and see if there's any practical problems. Second thing is to find a name for it. What would you call a box containing all the above?