As a software developer, I don't buy that as an explaination why it will cost extra. It can't be that complex and they sure as hell have both the menpower and the time (which won't be soooo long) to develop this feature for "free". Free as in "you're already paying 13$/15€ per month for this game already anyway".
Those guys already made the crossserver dungeon finder, AFAIK every server in the US is now in a single battleground pool, and they made you able to chat with other players on other realms and even in other blizzard games via RealID. Dungeon finder with a few fixed members from your realID list probably isn't far away from that code.
And Blizzard has a history of consistently putting out great games, with very few bugs (in comparision), great balance (again, in comparision) and with a very openended design that allowed countless of mods to thrive. The engine wow runs on has gotta be really well designed, seeing as how much they put into it over time with it still being amazingly stable. Very few features (especially once that use already established for free modules, such as realid and the cross-realm dungeon finder) are gonna be so hard to code into that without charging ADDITIONAL monthly fees for it to be worth it.
Guil Chat and Remote auction house with extra monthly fee? Well... it should really be free of charge already, but it does add a genuine new functionality and isn't needed ingame so I can kinda see why they charge for it (though again, should be free). But playing with friends on other servers WHEN YOU ALREADY HAVE THE TECHNOLOGY? Should be goddamn free without a doubt.
Good thing I haven't played in over half a year now