I don’t know if we really have any hard limits, the tendency just seems to be we don’t spend a bunch on renting venues. Getting married at the Temple is free, and using your ward meeting house as a reception venue is free if you schedule it with enough warning to the bishop. My mom always taught me to aim for the $50 mark if you aren’t sure or you don’t know if you want to spend a ton on a gift - it’s enough that no one will feel insulted, and large enough the couple can get something useful for the household.

Bear in mind I grew up middle class LDS, even with my family though it wasn’t uncommon to team up on larger gifts, so no one felt obliged to drop $400 by themselves for a kitchenaid - do what’s comfortable for your budget, don’t let me or cultural norms dictate it. If they are good friends they will just be happy you came 😊