Other than initial additional purchases (Collector's Editions, etc.) I have had the willpower to not spend money on games other than their subscription fees.
I tried Wizard 101 and Free Realms and Runes of Magic, and when, with greater frequency, I started to meet up with things that required a purchase I didn't pony up, instead I quit.
I admit the randomness of winning something through the Trading Card Game made it easy for me to not spend money trying to get a turtle mount or spectral tiger or even one of the easier things to get - various tabards. But I also give credit, on pun intended, to the fact I simply don't want to spend more money on a game I'm already spending money on.
I played City of Heroes a few months ago and was tempted to purchase the additional costume packs, because the original costumes had become dated since I first played. A big part of CoH is the costuming. It would be like purchasing additional quests in WoW. It felt wrong to have to pay for something that should be free. So I didn't give in to temptation.
Now Blizzard will be adding a store where you can buy a non-combat pet directly from them.
I don't think I'll spend any money on those either. There is a crack in my willpower but its sealed by the fact there are pets I still don't have in game. I'll just spend time trying to get those instead if I want a new pet.
I pay $15 a month for WoW. It's cheap entertainment but that doesn't mean I want to throw more money at it to keep it fun.
Back on PUBG
13 hours ago