Well it depends on what I get to do with Gates in that year and who pays for your living or if the money gets taxed. If we only have a coffee each day then even $500,000 post-tax (I am German) should be fine. If you get a chance to work in his organisation and to influence decision making with your ideas and contribution of thoughts (my ailing renewable energy start-up could really help his goals) then the year with him and the contacts you make shall be worth it — provided you do not need a million bucks to stay afloat and besides him during that year! For if you need to pay for that year with him it might be attractive to just take the cash and try to get famous by being the guy who rejected a year with Bill Gates for $500,000.
So again we see: Answers are never as straight forward as some short question may suggest if you switch your view from a simplified I/O approach to the reality of system-of-systems.