While you do not have to pay any money for the game, you can only get one legendary crest every month. Even buying one of the battle passes will be rewarded with the equivalent of two legendary crests every month. Then, you'll have to buy them on your own Diablo 4 Gold. Legendary crests range in price from around $2 to $3 for each. The number of gems you'll require to increase the power of your character's equipment especially when you consider the very low drop rates of five-star gems, is the reason the cost of maxing your character's abilities is so high in Diablo Immortal has been estimated between $50,000 and $100,000 or more depending on how deep you go into the system of resonance for gems. (Rock Paper Shotgun has detailed cost breakdown that falls on the moderate end of the scale.)
Diablo Immortal has been given an extremely rough time for this model of business which is perhaps not in the way it should in light of the fact that free-to-play games like Genshin Impact and Lost Ark have a lot of gacha-based mechanics that attract big-spending "whale" players. Diablo's popularity and fame with a large PC gaming community, accumulated over the course of a quarter century, is definitely one of the reasons. However, it's also true that this particular system is problematic, and the very character of Diablo games has something to do.
If you purchase legendary crests, you are not buying a single roll of dice like the case with a FIFA Ultimate Team card pack such as. You are purchasing a chance to play with the dice, to get into the game engine and tweak the drop rates (slightly) in your favor. These addictive gaming mechanics aren't isolated from the addictive gameplay mechanics. Instead, they are linked directly into combat and loot drops inside the game. Diablo is uniquely well-positioned to achieve this. As my colleague Maddy Myers pointed out, these games heavily focused on loot always had a slot-machine feel which Diablo Immortal's model of business makes the game appear as if it were cheap Diablo IV Gold.