We've all heard it. Since the inception of manosphere and maybe even long before it, this point has been driven into the psyche of every man to point out the disparity between the sexes. While most of the manosphere uses this maxim to make you invest in yourself to better your status (and thus increase your options), feminists and gynocentric individuals use this maxim to discourage you from becoming better and to ensure you accept the notion of female superiority via reproductive biology.
As with most things, these people do not want to expend effort to become better and instead work hard at creating the mythos that they are better by default. "Women are wonderful" effect is a very good example of this. People who subscribe to this effect do not tolerate any notion that women are fallible human beings who make mistakes and should be held accountable to those mistakes. They prefer to establish the notion that women are infallible angels and any mistakes they have made is because of some external source messing with their inner (good) compass. "Sperm is cheap, Eggs are expensive" is just another notion that has been hijacked to serve the interest of people who subscribe to gynocentric ideals.
This is also the reason most women want a finished product and not a man in development. To someone who is "born perfect", the notion of having to work at yourself to become better sounds inferior and "unnatural". Such people are not good relationship material because they expect things to work out naturally (and in their favor) and will not expend the effort to maintain a healthy relationship. This can be observed in the relationship patterns of women who are slaves to their tingles. Their utter failure to realize that you won't become an athlete (regardless of your genetic potential) if you don't train hard makes them and keeps them a poor prospect for any long term relationships.
So, the next time you hear or read a dismissive "Sperm is cheap, Eggs are expensive", remember that they are equating their entire worth to the reproductive process (and not their character) because they don't have a good alternative to offer. Understand where they are coming from and go find someone better.