If by extracurricular, you mean "office work", I wasn't doing any of that when I was copilot.
Well, again, it's contract. Like sometimes I'd have a 12 day trip that I'd get bought off of to train another pilot. So I'd still get paid the full amount and I'd just take the time off or pick something else up and "double dip". Generally I'd work 12 straight days (two 6-day back to backs or 1 12-day toward the front end of the month, then for the next month, block my flying together towards the end so I generally had blocks of 21+ days off.
It's been wrecked for the 2018 tax year, and
per diem is not compensation but I'm about $7K to $9Kin meal expense (IRS 2105, line 5) domestically whereas that number was over $20K most of my years as 7ER and 330 copilot.
Between trip efficiencies (in my eyes, probably not the companies), higher per diem amounts, lower expenses and the ability to pick up time, if I wanted when trips were dropped, plus "seniority shaq-foo" even making significantly more, hourly, as 320 CA compared to my stint as 330FO and especially 7ER FO, the difference in pay was negative, at first, relatively even then somewhat noticably higher a couple years after the upgrade. The ability to work big blocks got heavily modified under FAR117, however.
Pay Rates + Work Rules + Flexibility of Seniority is the name of the game at my shop. But I'll certainly gather that someone like
@ClarkGriswold is overall out earning and under-working the average captain his similar seniority.