Thanks to user Dave Harper, I discovered a great solution for ethernet over coax which is much cheaper than MoCA. It does have caveats, but is a great fit for HTPC and extender systems.
DECA is DirecTV's version of MoCA. But DirecTV DECA hardware is much cheaper than MoCA hardware, especially if you buy on ebay. MoCA is designed to work on the same wires as cable TV signals without interference. But DECA is right in the middle of cable TV frequencies, at 450-650mhz, which makes it impossible to run cable TV and DECA on the same wires. However, since HTPC users typically don't need cable TV signals anywhere except at their tuners and cable modems, the rest of the coax network can be disconnected from the cable TV network and be dedicated to DECA only. DECA bandwidth is 100Mbps full duplex, so should be sufficient for most video streaming needs.
Essentially, all you need to do is connect a powered DECA adapter at a router or switch and another powered DECA adapter at every extender. DirecTV makes several DECA devices to do this, including older DECA I receivers and the Cinema Connection Kit, or the newer DECA II devices. I used the latest DECA II devices because it was easy to find kits that included a power supply, and very inexpensive. So just buy one of THESE
DCA2PR0-01 kits for each node in your network. Older DECA devices and power supplies will also work fine.
Here is the DECA II kit needed for each node. At your switch or router, simply plug in the power, connect the DC/RF adapter then connect to the short cable attached to the DECA device, then connect an ethernet cable from the DECA device to a port on your switch or router. At each extender, connect power in the same manner, connect the other coax port to your coax network, and connect ethernet from the DECA device to your extender. 2 solid green lights on all DECA devices mean you have a good connection. Amber means you have a bad connection, so check your wiring.
If you are interest in power usage, my Kill-A-Watt says each of these uses 3W.
These adapters use Entropic MoCA
chips shared by many MoCA devices like Actiontec, ChannelMaster, Hirschmann and Wi3. The main difference is the different frequencies used by DECA.
Here's one vendor:
? ($10 each shipped from the mid-west)