UK standard visitor visa REFUSAL for an unemployed mother [closed]

I am an Azerbaijani citizen and I had applied for a standard general visitor visa for my mother which just got rejected today (for 7 days). I have applied for a Tier 4 visa as well, and am still waiting for the decision.

Since she is unemployed, she financially relies entirely on her husband. My father earns a decent amount of money and can support both of us in the UK (he will providing funding for my postgraduate tuition fees and maintenance in the UK as well). He has provided his salary slip, bank statement, employment verification letter, and a copy of his passport.

My mother has a house in her name. She has deposited money in her bank account. She has provided a letter of explanation noting that she had deposited the funds received for the sale of her house. She receives a certain amount of payment of interest from her deposit on a monthly basis.

However, to our dismay, she was refused based on the reasons below:

I still have no idea why our 7-day trip is going cost 90000 pounds to my father!

How to prove that she receives regular deposits that match her bank account? Moreover, how to prove that my father is financially able to sponsor us? Can anyone suggest what should I do and what sort of documents to submit if she reapplies? I am totally confused.

Thank you in advance!