Upvote:2
I have successfully sponsored a UK visitor visa application three times since 2017, all for the same applicant. The latest one (in 2023) was for a long-term multiple entry visitor visa. On each occasion I was paying all the costs.
The question about how much are you planning to spend is asking about the total planned cost of the trip. The second question is asking about how much your sponsor is contributing towards those costs.
As your sponsor is paying for everything, put the same amount in both sections of the application. You will need to demonstrate that your sponsor is willing and financially capable of providing the promised financial support by including his bank statements and evidence of income eg payslips plus a letter confirming his intention. The ECO needs to be convinced that the funds will be available to you when the time comes.
See also Should I submit bank statements when applying for a UK Visa? What do they say about me? and UK Visa Refusal: Provenance of funds/parking