@Andreas ...
I realize there could be pros and contras. Among the contras there could be (for example) many small relays overloading the dirauths. I would like to hear more about the contras.
A Pi running at its line speed isn't exactly a small relay.
Of course it isn't. A 20 mbps Pi relay has been reported here, still under-utilized.
...
Additional info about my experiment: I have just fired up an additional relay on Pi Zero. That's a fucking $9 Tor relay, including flash card and case. Looks like an oversized USB stick and plugs directly into a USB port of a computer. No need even for power supply.
Why wouldn't you run the relay directly on the connection/powering computer?
As I said, it is an experiment to see if this is working at all and what's the performance. Also, it was easy - I could use my PC to ssh into the Pi via the USB port, and am running a relay through the same port, so no tinkering with hardware. Eventually the Tor relay stick could be plugged directly into a USB port of a home router, I believe that there are some that have such ports.
Also, is the external USB network interface included in the pricing calculation?
What external USB network interface? Pi Zero has a micro USB connector. All that is needed is a standard USB cable, not even OTG one, I fished an old one from my junkbox. If you want you can add a whopping $1 to the cost :)
If you mean microUSB-to-Ethernet adaptor, that's $1.96 on eBay:
http://www.ebay.com/itm/1pc-Micro-USB-2-0-to-Ethernet-10-100-RJ45-Network-LA...