Depends on the game and how much I’m really into it. I tend to always wait for sales for stuff (and I also get PSN credit via cdkeys so I can effectively double up on savings), so I mostly pick up dlc in a sale. Games like Fallout, Skyrim etc usually have really vast, meaningful dlc (and I love those kinda games too). That said, I never bought The Witcher 3’s dlc because the base game was just so damn big in the first place, lol.
In all, I’m usually willing to spend around £5-£10 for dlc.