A very complicated question, since different families with the same income will have different needs. For instance, one may Ch”v have a child with special needs or medical problems that cost a good deal of money. An older parent may need help and care giving. And of course the everyday costs of life differ from place to place. Compare housing costs in Lakewood or Brooklyn with housing costs in OOT communities.
And again, different yeshivas have different budgets and different priorities. A yeshiva with a new building and a high mortgage will need a higher income than one in older or remodeled quarters, and the number of parents who can afford full tuition will make a difference too.
It would be a good idea if all yeshivas published a detailed yearly report on income and budget so parents could know exactly how much money is coming in and where it is going, letting parents feel informed instead of doubtful. For instance, doing remodeling or adding an addition instead of building a new building from scratch could avoid heavy mortgage payments and less financial strain on the school. It also helps if the school meets its payroll obligations in full on time. Financial strain on the teachers will prevent them from teaching at their best.
This is something that demands our immediate and serious attention, since the welfare of our next generation depends on how we solve the problems of our mosdos.