When is wasting money a sin?

I'm slightly confused about when wasting money is sinful and what is considered to be wasting money in the first place. Is it when we buy something we don't need? In that case we'd be sinning all the time. Where is the line between buying something and wasting money?

For example, I want to buy this $30 reindeer ski hat to wear on Christmas. It looks awesome and I'd probably wear it during the winter to keep my head warm. Would this be wasting money? I don't need the hat, but I'd continue to use it during winter, making it useful.

I'm probably being scrupulous, but for future reference: when is buying something we don't need, sinful? Is it when we have little, or no use for the item?

Thanks, hopefully I made a bit of sense