Those below the medium just don’t get married and have children.
Becoming a religious is not without cost, either. Seminaries don’t run for free.
The best course from a purely financial point of view may be to become a hermit or wandering mendicant.
In my experience, North Americans tend to think they need a lot more than they actually do in order to provide for their families. There are ways to make things easier financially, such as limiting new purchases and buying at thrift stores, making everything homemade (including simple cleaning products), living simpler in general, being willing to sacrifice certain comforts and wants, renting and not buying a home (the Jєωιѕн system has made it such that one never truly owns their home anyways). Families of 10+ used to live in tiny farmhouses just fine, even poor families. Women do not need much; if they claim otherwise, they are probably entitled and at least have a remnant of avarice.
According to what I can recall from St. Thomas Aquinas and the Canon law of 1917, religious houses are forbidden from charging aspirants an entrance fee, and are not allowed to charge a monthly fee for room and board during the noviciate, unless the religious house is extremely poor, but they are not to let that be a hindrance to receiving the postulant and ought to ask for donations. St. Thomas taught it was simony to charge a fee for entering a religious order.
St. Alphonsus Liguori does not recommend marriage unless one is habitually incontinent, simply because it is objectively more difficult to save one's soul in that state. St. Francis de Sales taught that marriage requires more virtue than in any other state. For religious life, he recommends that if one is unable to find a religious house that observes the holy rule strictly, it is better for one to stay home and live as piously and detached from the world as possible, than to live in a lax religious house.