The least expensive option for FC on UA or AA is to buy a coach ticket and upgrade using FF miles. On UA, you need 30,000 miles per ticket RT. On AA, you need the same number of miles plus you will have to pay a co-pay of $300 per ticket RT. While you can buy miles if you need them, the most cost effective way to get a substantial number of miles relatively quickly is usually to sign up for a credit card affliated with the airline. You may not be able to upgrade using miles on all flights, but you do not need to be an elite level flyer on UA or AA to upgrade using miles.
Another option is to purchase restricted FC tickets, also known as Y-Up fares. They are not as flexible as standard first class tickets, but may be significantly cheaper. If you can fly out of more than one airport, price them from each because the cost is not logical. For example, flying out of our local airport with a connection thru LAX was more than $500 per ticket cheaper than flying our of LAX directly.
Another option to get more legroom for the flight to Hawaii is to fly on UA and purchase Economy Plus Access (assuming you don't already have eltie status with UA). This will allow you to book your seats in the Economy Plus section, which has about 5" more legroom than standard coach. The cost for E+ Access (for a non UA elite) is $349 per year and it allows you to book you and a guest into E+ on each flight.
Best of luck -- Suzanne