onodrim

You can still talk about the ring, and still have it be a surprise. Just start the conversation with, "Let's say one day I want to get you an engagement ring, what do you like? etc..." That way, he'll know what to look for and he can still surprise her with it when he decides the time is right.

As far as how much to spend goes, well I think it's silly to have an "appropriate amount" myself. Find something you both would like and who cares what the cost is. The important thing is the meaning behind the ring, not the dollars put into it. But since you asked for help ... The generally accepted minimum is one month salary and the maximum is three.

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"The world is so exquisite with so much love and moral depth, that there is no reason to deceive ourselves with pretty stories for which there’s little good evidence. Far better it seems to me, in our vulnerability, is to look death in the eye and to be grateful every day for the brief but magnificent opportunity that life provides"
-Carl Sagan