Or....all the deeds (portals and otherwise) are magical, thus the cost of a single chair being equivalent to the combined annual income of a small village.
I'm not sure this has ever been a) asked or b) defined. I'm of the mind that it doesn't terribly matter, sort of like how all houses are much like the TARDIS. And it's not so much as establishing an actual portal as it is tapping into an existing network of portals with fixed effect. If a mage were to actually establish a fresh three-destination portal between Location A and Fort Wayfare, Prantz and Arnax, it would cost more than 4000 True in materials alone.
Anyway, deeds, regardless of end-result, are mechanical conveniences. If you consider they even truly exist in-game, then it can go either way. If deeds don't really exist and you're truly buying whatever it is the deed produces, then yes...you must make an assumption that a team of unionized, burly movers in coveralls come to deliver your new purchases.
Like I said, I don't think it's ever been asked or even considered, so I have no official answer to give.
