I imagine some demographic groups are probably more lucrative to advertise to, for various reasons (e.g., they have more money to spend, tend to spend more, etc

I imagine some demographic groups are probably more lucrative to advertise to, for various reasons (e.g., they have more money to spend, tend to spend more, etc.). So the less-lucrative demographics will accordingly be cheaper, which would appear to be a "discount". To go back to old-fashioned on-air TV, the ad slots after midnight were, I'm sure, much cheaper than the ones during prime-time.