Well, if I understand your question correctly, using "prepared" implies being ready and able to pay the tuition whereas "here' suggests to me that the person would be physically present to pay the tuition.

“ If you'd attended Stanford, I would have been prepared to pay (for) your tuition” Why did it use " was prepared to " here? I consider "prepared to" is also right, don't I? It's a plot from a TV series. A dad wants his/the daughter to go to study at Stanford, but his/the daughter wants to go to a NY college.

I found out that BE EXPECTED TO DO STH is the same problem .Am I right ? For example, Many are expected to learn well by parents .

This is a variation of a type 3 conditional. https://www.ef.com/ca/english-resources/english-grammar/conditional/ "If you had attended Stanford (University), I would have paid your tuition." (type 3 conditional) "If you had attended Stanford (University), I would have been prepared to pay your tuition." (type 3 conditional) "If you had attended Stanford (University), I was prepared (ready, willing) to pay your tuition." (equivalent to "I would have paid" and "I would have been prepared to pay") other notes Why it use " was prepared to " here? I consider "prepared to" is also right,don't I? should be Why does it use " was prepared to " here? I consider "prepared to" to be right. Why isn't it? It's a plot from TV series. Dad want daughter to Stanford,but daughter wanna go to NY college. should be It's a plot from a TV series. The dad wants his daughter to go to (study at) Stanford, but his daughter wants to go to (study at) a NY college.

This sentence uses the conditional tense ....so if you read through a grammar book the conditional rules you will understand