I've got an espalier Cox on my one bit of sunny wall. I bought it trained that way, for about £25, so it's a bit more than the £18 or so I'd expect for a standard Cox the same size.
Put up trellis or wire and restrain the horizontals. Prune (apparently) in JULY not winter, it's different for these fellows. It's a good way to green up an ugly wall too. No idea on productivity yet as this is its first year.
Previous places I've been at have had those compact vertical "Ballerina" apples. They're really rather good - they'll grow in a (big) pot too. They really do fruit heavily for their size. One tip, when pruning try to thin 'em out as the interior can get cramped and stifled, which leads to poor air-flow and mildew, etc, unless you keep on top of it. Great producers for a small garden.