I've not done it, but I did look in to it, both for my current V90 and a previous S80 - in both cases the quote came in around a grand!! Roughly double the cost of ordering the same from factory!!!
Cost to dealer would be minimal in the greater scheme of a car purchase....if I were you I'd be trying to get them to do the mod as part of agreeing a deal on a Selekt Used XC90.