Applying UK Visitor Visa for my Parents

UK Immigration Rules don’t specify a minimum savings/income requirement. https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules What matters is that an applicant has sufficient funds available to them to cover the costs trip, and that those costs are proportionate to their overall financial situation.
One cover letter should be sufficient. Your mother’s application is dependent on your father’s, see How should a family with young children apply for UK visas? for advice on how to link the two applications
Enter the planned cost. If you’re paying for everything the amount you enter here should match the amount in the later question about how much the sponsor is paying.

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Source: stackoverflow.com