I say they have gone up in price, I don't remember paying that much.
I would not use them on any surge protected extension as the the protection could degrade the signal from unit to unit. That's probably what the gurus mean when saying they should not be used on extensions.
One other thing to think about is where the Ethernet lead plugs into the homeplug. With the Develo this is at the bottom of the unit. All others that I have seen plug in the side, even my Netgear one plugs in the side. This means that 2 sockets for the one unit. It is impossible to use the socket at the side where the cable is. Of course if there is nothing to be plugged into a socket at the side and there is little clearance at the bottom, of the unit then a side Ethernet socket might be what you need.
As things are at the moment it is cheaper to by a pair rather than a single unit. That's why I suggested not to buy the cheapest if you see what I mean. The reason is if one fails, you would need to by another pair as single units are hard to come by.
The only ones in your links that I know personally are the Develo units. Reliable, take up one socket each and come with software for setting up ones own encryption code if one wants to.
Pass thru filters I know nothing about so I couldn't comment on.
I trust I am being helpful here.