To get a rough idea of prices I know of a car upholstering company based around the bournemouth area who charge around 150 for a full wheel retrim. It's pricey but have seen pics and the quality looks very good. I'd expect this to be the most you'd pay for a basic retrim though, reckon you could get it cheaper elsewhere or as someone said find a non car related tradesperson who could do it.
Sometimes this is lots cheaper than a new wheel, my Clio wheel is made of really soft leather and it just gets hacked to hell. A new wheel is around £350 from Renault, but am getting a friends brother who works as an upholsterer to do it a bit cheaper, looking to pay about £75 still though!