Ok... I start out with asking the obvious. The tank is plastic, so why would it need to be replaced? It can just be cleaned out and it would be perfectly fine again. Also, even if there were rust in the tank... Like it was stated earlier, the major particles would be caught by the screen on the pump inlet and the lesser particles should have been caught by the fuel filter, unless they were large enough to tear a hole in the filter media...
I would strongly recommend finding another dealership as this one doesn't seem competent enough.
Even if you needed to replace your fuel pump, $2k sounds like a lot!
GM Parts Direct
Fuel pump: P/N 25326335 - Dealer $441.98 - GMPD $220.99 - Labor 1.1hrs
Fuel Injectors: P/N 24508208 - Dealer $115.36 ea. - GMPD $57.68 - Labor 1.3hrs for all
Fuel Filter: P/N 25121293 - Dealer $15.60 - GMPD $9.34 - Labor 0.5hrs
Fuel Pressure Regulator: P/N 24506988 - Dealer $89.17 - GMPD $44.58 - Labor 0.6hrs
That would basically cover every part of the fuel system that would be affected by the rust particles if any of it did infact get past the fuel filter. I can hardly believe that they would actually pull the fuel tank from the vehicle to inspect the inside for rust seeing as there is an access panel in the trunk where the fuel pump can be pulled through and a nice 3-4" hole that can be looked through to inspect the inside of the tank. It would be completely possible to clean the entire tank out from that location (I believe anyway).
I still can't believe that the injectors would have seen any of the rust particles... Also, if you are at all mechanically inclined, this all is fairly easy to replace (the injectors being the hardest of them all IMO - not including the tank because it can be cleaned!).
Now if they did have to replace the fuel tank for some reason...
Fuel Tank: P/N 25744826 - Dealer $466.73 - GMPD $275.37 - Labor 4.7hrs
that would be where A LOT of cost would come from... But there is no excuse to explain why they can't clean the tank in the vehicle!
**All labor times were recorded from AllDataDIY.com