not much, actually. new Macbook Air is $949, a new iMac is $1149. you can get a good, large monitor for $200.
But isn't that an apple and an orange? The basic MacBook Air and the basic iMac are quite different in terms of memory, storage, processor, et cetera. What's the cost of the equivalent MacBook Air?
I think you mean MacBook Pro, but I get your point. The basic MacBook Pro and the basic iMac cost the same, so yes, there would be an added cost for a large monitor. however, it's not that much, and the point of the article is that mobile, portable machines with a common OS seem to be the coming thing. there will likely always be a market for those that want (dare I call it) a brick-and-mortar desktop, but having the flexibility to grab your computer and take it with you, and just hook up to a big screen when you need it seems to make sense to me. I don't think it will be long before tablets and phones compete with laptops and desktops on a processor/memory level. I'm sure that bluetooth (or similar technology) monitors exist, and just by placing your tablet or phone in proximity to them they become the main monitor.
