[ZeroSync]

For the top groups, Spam is extremely profitable.

A lot of them were commissioned to spam knockoff viagra pills, or other pharmaceuticals and they take a % of all conversions. Hundreds of millions in total/yearly.

Other spam is outright scams as well or efforts by cybercrime groups to recruite money mules etc.

So I suppose it depends what type of spam you're referring to but as an whole in cybercrime its one of the oldest and most profitable.