Depends on how old the game is and how big the DLC is IMO. Rimworld, for instance, has quite a few DLCs now, but they are all well worth it if you like the base game. OTOH if a game just has cosmetic DLC or the DLC is coming out super near release that's a red flag.

This describes just about all Paradox games made in the past 15 years, sadly. They release with a barebones concept, then slowly drip-feed content for 5-30 bucks a pop, each one usually sitting at "Mostly Negative" because it doesn't fundamentally add or change anything most of the time, and the times it does- meh. Crusader Kings 2 was my bread and butter for a long time. Played Crusader Kings 3, and it felt like almost every helpful mechanic that existed in CK2 was stripped, and then added on again over the course of years. It was so infuriating, that I just don't buy their titles anymore.

Vampire Survivors is an exception.

At the very least, you can still pirate it and play cracked multiplayer with friends.

I made the mistake of buying the game year ago, and bought a bunch of DLC at 50% or greater sales, and now the sunken cost fallacy has taken hold on me, and I still want to buy more . . . . (at least I'm broke so I can't right now hehehaha)