I've just installed a wall hung back to wall WC in my downstairs loo. It's great. Here's what I learned along the way...
If your requirement is for the cheapest then you will end up with a floor standing one as wall hung are all more expensive. It could be that they are more expensive to make but I suspect that really it's because they are considered more "exotic". Certainly the full cost, including all the fittings, will be higher as you will almost certainly need a frame/bracket. In my case the requirement was for quality but obviously I didn't want to pay more than I needed to.
The frame was the easy part and my local discount plumbing merchant got me an all singing Geberit frame complete with cistern for about =A3160. These things are so simple to fit that even a plumber couldn't get it wrong :-) They bolt to the wall and the floor and stick out about
160mm from the wall (plus whatever decorative facing you use). Future servicing is all done through the flushplate aperture so you can afford to build it in completely. The overflow is into the pan so no overflow pipe to worry about. It is dual flush and in use it is almost completely silent. Gerberit publish full details, including fitting instructions on their website.
The pan itself was more of a problem. All of the UK suppliers I could find (and I searched long and hard) were taking the p*ss. They wanted at least twice as much as I could pay by buying it in Germany and for the particular pan I wanted the best I could do in the UK was 3x the price. In the end I ordered it from megabad (.com) and even with the shipping cost I still saved around =A3100 to =A3150. In Germany wall hung is the norm and I think that shows in the price the suppliers can get away with charging.
Fitting a wall hung is stressful. Very stressful. The whole thing is supported on two 13mm studs (blots). These need to be tight enough to support the heaviest load but you've no way of knowing how tight that is. On the other hand too tight and the first thing you'll know is a "tink" as the ceramic pan cracks. Fun.
If you do fit a wall hung make sure you fit a foam gasket between the pan and the wall. If your wall is ceramic tiles you really need it as they won't be perfectly flat and any high spots will put a lot of pressure on those points of the pan which press against them. Even if your wall is completely true as mine is (Mermaid wallboard) the foam allows for slight movement between the pan and the wall and stops the thing creaking as you sit down.
One final note, the new "easy clean" coatings really do seem to work. Our pan is made by Duravit who use a coating called Wondergliss. I was sceptical but paid the extra anyway and I'm now a believer. How can I put this without being indelicate? You don't need a loo brush anymore...=20
Good luck, Calvin