How can I have a gambling system for fighting pits that the players can bet on?

In my current campaign there’s a fighting pit in the town the PCs are in. I want the players to be able to gamble on fights here in a way that feels ‘real’. The easiest thing I could come up with is equal odds in a fight, double or nothing rewards.

I don’t want to spend a ton of thought power managing this. Has anyone executed something like this in a campaign before?