You are obviously willfully misinterpreting what the OP meant by "cost".
You say "the thought is what matters" - this is 100% true, and "the thought" has a "cost". It "costs" an hour of sitting down and thinking of what to write to express your feelings to someone. That's what he is saying is "proportional" to love.
It "costs" you mental space to love your mom, and that can definitely happen with $0 entering the equation.
And with respect to "extra worth on people struggling to do something meaning more love" - if you spend the time to sit down and write a poem, when that's something that you don't excel at, someone will think: "oh wow you clearly really love me if you spent the time to write a poem, I know this because I know it's not easy for you and so you must care a lot to have done so anyway". If you can't see that... thats... ya