For almost all DLCs apart from the really new ones, you can find keys for 2-3$ per DLC and half of them are unnecessary radio stations anyway. I revisited the game last year and had no DLC puchased ever, I spent about 15€ in total to get all the major DLCs up until then. Only for the DLCs and Content Creator Packs that came out after that I paid 10-15€ each.
Some of them may also be available for <5€ already and the Content Creator Packs could be replaced by free workshop assets if you want to save more money.
It's also worth noting that the game is 8 years old already and still getting new content.