Do you have documentation of the difference in cost between coach and first class at the time you booked? Many of us recommend taking screen shots of the fare grid whenever purchasing first class, for this reason.
Also, is this just one leg of a multi-leg journey to your destination, or is this a simple one-way/round-trip?

If you don't have documentation, you can try providing us your specific flight details, the date of original purchase, and the fare you paid, and someone can use a service like ExpertFlyer to determine what the fare difference might have been.

After you have the documentation, your steps are to (1) contact AA, tell them the $75 is insulting, and request the fare difference returned to original form of payment, plus some type of "customer service gesture" because of their insulting offer, perhaps 5k miles or a $50 voucher; (2) File a complaint with the Department of Transportation; and then if all else fails, you could (3) sue AA in small claims court, which is relatively easy to do and would likely result in a settlement.