I think RBMcdonald accidentally added a couple of zeros to the last number there. The biggest win possible on a contract is $75,735: $850 invested in 85,000 shares @ $0.01/ea gives a payout of $85k but $8,415 fee (10%) is subtracted from the $84,150 profit for a net payout of $76,585 ($75,735 profit.) Amounts near such theoretical max have been claimed in the comments of some markets but I haven't been in such a fortunate position myself.
No, the $850 limit only applies to what you put into the market, not take out. So theoretically If a contract is available for $0.02, you can buy 42,500 shares (assuming that many shares are for sale at that price (unlikely), then if the contract resolves to your favor (again, very unlikely), you would receive $4,250,000. I suspect if this ever occurred, there would be a long hard look at it, but it would not be in violation of the $850 limit.