iPhone 7 Plus
I would like to know how much it would it cost to replace the mic
You would most likely need to pay for an out of warranty replacement. To my knowledge, the microphone is not an individually replaceable item. An out of warranty replacement iPhone 7 Plus will cost (in the US) $349.
I would give more serious consideration to getting a newer more modern iPhone for that much money personally.
See this link --> iPhone Repair - Official Apple Support