It depends on the builder - I've found a few now where, rather that being soldered onto the motherboard, the AC plug is actually on a little cable lead.
Like this one:
To replace the plug, you still have to tear the laptop completely apart (In the case of the Toshiba L645, it took me about an hour from start to finish, but I work slowly) but for the customer, an hour of labor and a $7 wire is cheaper than an hour of labor and a new motherboard. (!)
However, most laptop repair places will, like Blain, assume it's attached to the motherboard and quote you accordingly. (And rightfully so - it's true in the majority of cases.) You should probably find the tech manual for your particular laptop and verify whether it is, in fact, a cable or not. Then you can negotiate from an informed position.