Best of 5. Make them earn their money.
No... Not for the prize money. Best of 5 because it's a better contest and it's more likely for better player to prevail.
It should be a meritocracy and prize money should be based on gate receipts, TV contracts (based on ratings), sponsorship, etc... I don't care how many sets they play. They earn whatever people are willing to pay to watch them play. I can play a thousand sets of tennis and no one will pay to watch me play.
Too bad the grand slams for Men's and Ladies are not held on different weeks. It should then be obvious what prize $$$ each deserve.
The USGA held both the Men's and Ladies Open Championships at Pinehurst 2 this year 2 weeks apart. The purse for the Men, $9M. For the Ladies, $4M. Fair and equitable IMO.
On the flip side... I would also argue women deserve more $$$s for Figure Skating championships. Meritocracy.