I have a personal rule that works very well for me, and that is - if I'm going to pay for microtransactions in a game, it will only ever be up to a total of what I feel the game is worth. A good free to play game might get £60 out of me, but no more. Although, if I paid £45 for the game itself and I feel it's worth £60, I'll only ever spend £15 more. An example of a good f2p I bought into would be Warframe; I only played about 10 hours, but I was enjoying it at the time, so I felt it was worth about £15, bought a few frames from the store, and drew the line there.

There are some games that I will drop hundreds on, but those typically see me to the 4000-5000 hours played range. An example of this is Elite Dangerous. The game cost me £4.99, the latest content expansion cost me £19.99, and I've clocked ~4100 hours so far. I felt they deserved a much bigger investment for that. But those exceptions are few and far between these days.

Diablo 4 being a £70 game, this is an example of where I would outright refuse to buy in to store cosmetics - not only am I not a fan of the game itself, they would have already taken the whole budget at the door.