כל מזונותיו של אדם קצובים לו מראש השנה עד ראש השנה חוץ מהוצאת שבתות והוצאות י"ט והוצאת בניו לתלמוד תורה
The income of a person is fixed from one Rosh Hashana to the next except for what he spends on shabbos, yom tov, and the Torah education of his sons
From this gemara we see that whatever a person spends to teach his sons torah, Hashem will pay back and is not considered part of his income. In other words, if his income was set at $x and he paid $y for his son's torah education he will still end up with $x dollars, the same would apply if he spent $0 on his son's torah education he would still end up with $x. This would seem to imply that tuition payments for boys do not take away from a person's income. Of course, the Gemara was talking about Torah education and since tuition includes payment for secular studies as well that would not be included. In addition, the Gemara only talks about boys where you have a chiyuv to teach them Torah, however, by girls since there is no chiyuv the money spent would come from your income.
This is just food for thought. This gemara should make everyone think twice about how they view their income and their plans to increase it.
I see that Gil has posted about this topic as well Yeshiva Tuition. He refers to an article by RHS Straightening Out Our Priorities where he also brings down this gemara. The whole article is well worth reading but I will quote RHS conclusion.
On a communal level, we have lost our bearings regarding what is a normal and proper lifestyle, and what is an opulent and improper one. In that context, some Orthodox people spend large sums of money on non-essentials without making yeshiva tuition a top priority, and consequently want to send their children to public school to save money. We, too, need Moshe Rabbeinu’s rebuke! What an unfortunate confusion of priorities! Our children are immeasurably more valuable than our homes and all other material possessions. If we really believed G-d that the Torah is the “kli chemdah” (Avos 3:18), and that observance of the mitzvos is the wish of the Creator of the world, how could we possibly be so lax regarding the Torah education of our children?