100bucks as the guaranteed prize is good amount already and I believe it is just so small amount of money for Stake, but it would be better if there is minimum bet requirement to win this 100bucks as if there is no minimum bet then there will be so many bots run by people with multiple accounts that betting with zero values.
same . i feel that its too little for a big event like that . i see that the winner of 10billionth bet only won 0.001btc because his bets are under 1000k sats but that is better than nothing at all , at least its almost free since he is only betting small amounts of sats and he can play the game without loosing those sats if he will only use higher chance of winning ( ex . 98% ) . but for a high roller , they can win at max 1 btc which is already a verry good prize .
Maybe your statement was right if you think since the bet amount is too low and because that the winner got approximate 0.001btc this is not easy to get if you bet on small amount atleast this is free after all.