Most seminaries demand about 15% when you get the acceptance letter, around 02/15, due by 03/01 to confirm the acceptance. The balance is due in 3 installments. Usually around 05/15, 08/15, and December or January. There are various schalarships that kick in at various times during the year, if you’re eligible, and frequently that last payment does not need to be made.
If there is an issue, that last payment is just about the only leverage that you have. I know someone close to me that had an issue in mid cheshvan, and the seminary was mochel the last payment. But they still got about 15K for less than a month’s service provided. (There was very little going on in the last week of Elul when the year began, and Tishrei which was already vacation.)
It is also not completely true that when they give you a spot they have lost the opportunity to sell it. It is well known that issues sometimes come up, and a (small) percentage of girls switch in mid year. So an opening can be filled in mid year.
Generally there are halachos about backing out of a deal in the middle. (Poel yachol lachazor bachatzi yom.) Under those circumstances, they should not be able to charge you the entire fee. Unless it is stipulated in the contract in the beginning. (Which these days, most are.) Then the klal of kol sheb’mamon t’nao kayam would apply. Meaning basically, if you agree to their terms, you’re stuck with their policies. The seminaries know they can get away with putting that line in, because by the time you get to that point, and you have an acceptance letter in hand, who’s gonna tell their daughter they won’t sign up because of some t’nai in the fine print?