MX-5 Exhaust Manifold.

Please bear with me because I'm a total dumbass and rather angry right
now, lol. So, I bought a 1990 MX-5 last year and I discovered it's
too impractical for my needs and decided to sell it.  A lady just test drove it
and complained about the noise. I told her she could change the muffler as
I thought it was an aftermarket one because the person I bought it from (a
mechanic, no less) has a few cars with purposefully loud mufflers. She
passed. I just emailed him for specifics and he said it isn't aftermarket, it
has a leak in the exhaust manifold.
So, how much should I expect to drop the price and is there anything I can
do about it for minimum pennies? Or am I screwed? I certainly don't want
to give him the job of repairing it.