Love is somewhat related to cost, but "proportional" is definitely not the word you want.
If love is proportional to cost, then rapists and psychos who kill their SOs are the true lovers since the cost is 20 years of jail time to life sentence. Do you want to live by this standard?
Actually you prove my point; the psycho loves himself so much that he will risk prison to get what he wants (or keep others from having it), but he doesn't love his SO enough to pay the cost of letting her go.
If love is proportional to cost, then rapists and psychos who kill their SOs are the true lovers since the cost is 20 years of jail time to life sentence. Do you want to live by this standard?