I've had 5, totalling ~660 coins. It doesn't show you the exact amount when you kill it - you have to visit an emporium to see your full amount and remember how much you had previously.
One was on Old Faithful during TT8, another was also in the tunnels on TT9. I've been mostly doing TT for the past few weeks.
As far as I have understood, whenever a regular skelly spawn is triggered, there is a 2% chance that one of these will also trigger. The entire crew gets paid.
The notification shows coins on the left of it, proportionate to the amount it paid.
The ~150 ones just show a small pile of coins, whereas the higher value ones show a bag of coins, or two bags of coins.