This isn't making sense to me. If in fact a gasket is blown, well then, that gasket seals the two parts that come together. That's why a gasket is there---to accomodate the design of the exhaust system into its various parts. It allows individual parts of the system to be replaced.
So if it is just the gasket that needs replacing, and if the gasket (being a gasket after all) is not welded to, or part of, any other component---then why does any other component need replacing.
Now, if they are saying some other component is DAMAGED (rusted out, broken flange that goes over the gasket and seals it, etc.) well then, that's a different story.
Unless they can SHOW YOU a damaged part other than the little gasket, I wouldn't buy this story. These chain operations (as a gross generalization) are not immune to over-selling product.
Get a second opinion. Go to say a muffler speciality shop that works on hot rods or race cars, or go to an independent garage that specializes in fixing imports.
I can't tell you about how much it should cost because I don't know what part they are attempting to replace. If they want to replace LEFT and RIGHT catalytic converters, yeah, that's about $1400 list price for the parts. (but you can shop around and do much better than that I think).