While avoiding the "should this idea be implemented" question for a second, I'm wondering about the engineering issues here: is it more efficient to pay men for vasectomies or women for tied tubes.
Women are subject to meaningful rate limits. Ultimately the number of women determine the rate at which a population can reproduce.
A man can mate with many women, so eliminating a single man from the reproductive pool might have a significant impact. However, the women that man could impregnate might simply end up impregnated by someone else.
Not entirely sure how to model this.