Note: Shafted = pay 0 cents.
Suppose that the robbers are named by letters, with the oldest robber in each scenario being Robber A. I'll list the robbers in order of youngest to oldest for each case:
In a two-robber case (John, Robber A), John always keeps the cash. Robber A has no leverage whatsoever, since he'll never have superior force to remove the captain. So, we would say here that Robber A would be shafted because John could win optimally by giving him no money whatsoever.
In a three-robber case (John, Robber B, Robber A), John only needs one vote. Robber B wants to get rid of John so he can be in the advantageous two-robber case. Robber A wants to AVOID that, so the John easily buys Robber A's vote with one cent -- the status quo is maintained, and Robber B gets shafted.
In a four-robber case (John, Robber C, Robber B, Robber A), John still only needs one vote (a tie means no superior force, which means the status quo is maintained and John wins). We again will ask our standard question: "If John loses the vote, then who gets shafted?" -- well, if the captain loses, it's a three-robber case, which means Robber B gets shafted. So the John easily buys Robber B's vote with one gold coin, and the status quo is maintained. Result: Robbers A and C would be shafted, B would be given one cent, and John would get the rest.
In a five-robber case (John, Robber D, Robber C, Robber B, Robber A), John needs TWO votes to avoid losing power. So we ask ourselves, once again: "If John loses the vote, then who gets shafted?" -- Robbers A and C (as determined above, in the four-robber case). So, you’d want to pay Robbers A and C one cent each to make them vote for you. Therefore, robbers B and D would be shafted.
In a six-robber case (John, Robber E, Robber D, Robber C, Robber B, Robber A), John needs two votes to avoid death. So, we ask, "If John loses the vote, then who gets shafted?". Well, as determined above, it would be robbers B and D shafted. So, you can easily win their vote by giving them just one cent. Therefore, Robbers A, C, and E would be shafted.
In a seven-robber case (John, Robber F, Robber E, Robber D, Robber C, Robber B, Robber A), John needs THREE votes to avoid death. So, we ask, “If John loses the vote, then who gets shafted?”. As determined above, A, C, and E would get shafted. So, you’d want to pay A, C, and E one cent each in order to get your vote. Therefore, robbers B, D, and F would not be paid at all, or in other words would be shafted.
In the desired eight-robber case (John, Robber G, Robber F, Robber E, Robber D, Robber C, Robber B, Robber A), John needs three votes, yet again, to avoid death. (Reminder: This is because he will obviously vote for it, making it (1+3)/8 = 4/8, and as described earlier, a tie means no superior force, which means the status quo is maintained and John wins.) So, we would then ask, “If John loses the vote, then who gets shafted?”. As determined above, robbers B, D, and F would be shafted if he lost the vote. So, you can easily get their vote by simply paying them one cent each and John can keep the rest. Therefore: Robbers A (the oldest), C, E, and G would be shafted; robbers B, D, and F would each get one cent each; and John would keep the rest.
So:
John: $7,999,999.97
Next youngest: $0.00
Next youngest: $0.01
Next youngest: $0.00
Next youngest: $0.01
Next youngest: $0.00
Next youngest: $0.01
Oldest: $0.00