I am very surprised that the dealer told you a replacement hull was within $200 of a new boat since the Oasis costs $2850 and includes 2 mirage drives at $500 ea. I really suspect he was just being lazy and just looked up a number in the catalog and never contacted Hobie service at all.
Lets just say for discussion that the rest of the components (ie seats, rudder, hatches, etc) are worth around $500 (just a wild guess). I have received replacement hulls from Hobie before and typically you can't purchase just a raw hull, all the hulls go thru the same production lines so any replacement hull will have the hatches installed, the rudder and all cables installed, basically a complete hull less the mirage drives, paddles, and accessories. I would be an utter nightmare to try to remove the hatches and rudder lines from your old boat and try to apply them to a raw new hull since there is machining involved, jigs, fixtures, and custom fitting of some components (ie.. like the gasket around the big hatch) so it would be impractical and useless for you to just get a raw unfinished hull.
Depending where you are in relation to California (where the Oasis is made) correlates to the shipping cost. I'm not certain how much it would cost to ship a hull from California to where I live in Florida, but can't imagine it being less than $200-$300 dollars shipping.
Having pictures of the problem area will help quite a bit (jpegs on a memory card) if it were me I would take the boat back to the dealer and have them call Hobie and send the pictures along with an explanation of the problem. Your dealer is your contact point to Hobie and he is supposed to take care of everything for you (getting replacement costs, shipping info, delivery, etc).
He may be unaware of Hobies procedures, basically dealers have direct contact with service, and between the two of them they will come up with a solution. This situation is not something he can just look up in a catalog, he has to contact Hobie service directly. ( again I suspect given the price he quoted you he was just being lazy and just looked up a price in a catalog somewhere without contacting Hobie service). It's his job as a Hobie dealer to deal with these situations properly, that's the way their entire system is setup.
Hope this helps you, let us know how it goes