If you don't spend any money on the game, you will just get one legendary crest every month. Buying the battle pass will be rewarded with two or three additional legendary crests every month. In addition, you'll need to buy them in person. Legendary crests are priced somewhere between $2 and $3 apiece. The massive amount of gems buy cheap Diablo IV Gold that you'll need to increase the power of your character's equipment given the extremely low drop rates of five-star gems, is the reason the cost of maxing out your character for Diablo Immortal has been estimated to be between $50,000-$100,000 -- and perhaps more when you dive into the system of resonance for gems. (Rock Paper Shotgun is an extremely thorough breakdown of cost which is on the more sensible end of this spectrum.)
Diablo Immortal has been given an extremely rough ride due to the business model it is based on and perhaps in a way in light of the fact that free-to-play games like Genshin Impact and Lost Ark aren't immune to similar gacha mechanics that entice high-priced "whale" players. Diablo's reputation and popularity among the core PC gaming population, built over a quarter of a century, is certainly an important factor. However, there is also a fact that Diablo is a particularly problematic system, and the very nature of Diablo games could have some connection to that.
If you purchase legendary crests, you are not purchasing a roll of dice like you purchase the FIFA Ultimate Team card pack or a FIFA Ultimate Team card pack, for example. The purchase is an opportunity to play with the dice, to reach into the game engine and alter the drop rate (slightly) in your favor. Gaming mechanics are addicting and are not isolated from the addictive gameplay mechanics, but rather tied directly to combat and loot drops inside the game. Diablo is incredibly well-placed to achieve this. As my colleague Maddy Myers pointed out, these games heavily focused on loot always had cheap Diablo 4 Gold a certain slot-machine-like quality which Diablo Immortal's business model renders real.