I think he's saying that so the other bitches would back off bullying Tia--
If the Prince shows no favor towards Tia and directs it towards Moira instead, the aggro would turn onto Moria 'cuz they'd think she's now the top competitor for the Prince's heart.
Betting the gift would also mean that other people would find the gift of little importance since he readily agreed. But it's most likely he believes Tia will win and people will less be suspicious and aggressive towards the gift he (ehem-- especially) prepared for Tia.
Prince is a cunning tsundere~ ╮(￣▽￣)╭
I think the prince wants aristia to lose because he hopes that her defeat will make her want to become a knight and so she will have no other choice but to become his fiancée.
Why is he saying that? Does he really want that princess to win? If not, he should be honest.