They made thousands of Sterlingworth in 12 gauge. There is absolutely no reason to pay that much for a shooter. A solid 12 should be in the $600-750.00 for a very solid shooter. Unless it is very high condition you should either wait until you come across one in this price range or be willing to over pay. Looking is more fun than buying because as soon as you buy you are either out of the looking/buying game or a better and cheaper one will come along.
Now on the other hand a 16 or 20 will set you back several times this amount for the same condition. Fewer made and demand is much greater for small bores so the price is much higher. Funny thing is that about 10 years ago 16's were hard to sell and went for less than a 12 did.
I think that the next big increase in prices will be on Ithaca small bores. Especially in the NID's. Prices are going up fast around here and I do not see an end in sight.