The winner is selected from among the finalists by telephone ballot, and it is difficult for the market makers to get into the minds of those who are to decide the result. In the final analysis, Maria Callas did indeed win, and by some margin. Although this proves nothing in itself, it is clear that people who switch on half way through the event are unlikely to vote for someone who performed at the beginning.
Batting order is certainly something to bear in mind when betting.
Each person only gets one vote, so if many of the acts are similar to each other, the votes for people who like that sort of thing get split. This year's Eurovision song contest was won by a couple of guys with guitars. Similarly, Maria Callas was the only voting option for people who wanted something different from the standard formula of someone dressing up as a famous pop artist and performing an inferior version of his or her most famous song. If most of the public are voting for a specific contestant, they are probably betting on the same act. Anyone who waited to see how the market moved on each eviction and lumped on to the same side as everyone else would have made a fortune.