Yeah, those are expensive little critters. I had to replace the ones on the M5 when I had to drop the exhaust to install new seals in the transmission.
It's insane to pay almost $90 for 6 nuts. You could use regular nuts, but after they are fried onto the exhaust stud they would be impossible to remove without damaging the stud. Just re-use your old ones.
Don't forget to use a liberal amount of never-seize on the new nuts and studs, this will help tremendously when you have to remove it again.