Having successfully sponsored a UK Visitor visa several times, I recommend that you complete the application as below.
In the Income and expenditure section of the application the following questions are included (amongst others):
- How much money are you personally planning to spend on your visit to the UK?
and
- What is the total amount of money you spend each month.
For Q1, enter the estimated total cost of the trip (flights, spending money, excursions etc).
For Q2, enter the monthly amount your mother spends in Pakistan.
If the estimated cost of the trip is more than your mother has in savings, and/or your father is helping her pay for it, answer ‘Yes’ to the subsequent question asking if anyone else is paying towards the cost of the trip, and state the amount being contributed.
Note that, unlike for Schengen short-stay visa applicants, the UK does not stipulate a per day minimum subsistance amount. This question Should I submit bank statements when applying for a UK Visa? What do they say about me? explains more about how UKVI assess an applicant’s financial capacity.