How much will it cost to get an ECU software update?

No, it's not the warranty. My car has been riding rough and misfiring badly for a while. For the longest, whenever I got it scanned, it would return a P0300 code. I've had it tuned and even replaced the catalytic converter but nothing helped. I got it scanned by a G-scan (I think that's what it's called) when it gave the P1779 code, and this was the only code that came up. I called a dealership and they told me they would charge $97 for the update.