Ignoring what else the money could do.

Money spent gambling is money you can’t spend on something else.

That alternative might be savings, paying down a debt, a trip, or another night out. It doesn’t have to be a perfectly optimized investment.

Entertainment is allowed to cost money. The useful question is whether this is still what you want to spend on, given what it replaces.

A few small bets may feel separate from a larger goal. Looking at a month’s spending can make the trade-off easier to see. The same is true of time: following bets all evening means that evening isn’t available for something else.

This is what “opportunity cost” means. The cost includes the option you gave up, even if the app never displays it.