200Mbit single units by TP-Link can be had for around £25.
Really it's a bit like WiFi, there are powerline standards such as there are WiFi standards. And then some manufacturers such as Belkin with their "HD" kit OCuk sell here (a very good price btw) who (I assume) provide the base standard and then over-ride it with their own as well.
As has been said, what you get will depend on your wiring quality and setup, however in a house, with a couple of 200Mbit powerline standard devices, i'f be surpised if you saw much less than 100mbit between them.
We have 4 TP-Link 201 series in our flat and 1 200Mbit Devolo device, they have all been given their own socket (quite important), one goes into the router (the devolo), one into the lounge, and one into each bedroom. There is a switch on the lounge and a PS3, Wii and laptop all go into it. The Devolo and TP-Link all play nicely together as they all meet standard powerline specs. From my research the only ones to avoid are some older Netgear ones as Netgear rushed things and ignored standards to be first to market.
So we really are taxing the technology, but (bar some frankly shonky setup software from TP-Link), they were all setup in 5 minutes and while i'm sure the network is collision heaven, if I look from my PC, the software connects locally to my device, which then finds the other adaptors and estimates a throughput, the lowest is to the lounge, which is the furthest away from me, and is estimated at 80-100. The rest range from 150 to 200.
Pings are great across the network and really you couldn't tell it apart from a standard ethernet setup tbh.
I'm very impressed by the technology!