The value of a courtship gift is its price.

So a self-written poem is worth less than a purchased fortune cookie?

I agree with your sentiment, but the retort would be that even a few minutes of your time has more monetary value than that cookie.

The diamond cartels love this kind of thinking. It should be about value, not price.

With a diamond, you are not getting value (except in some rare circumstances involving unusual stones).

It is not a good thing for courtship gifts to have value; if they have too much value they are likely to be dishonestly accepted by someone who isn't interested in the giver but does want the object.