Post-Grant Award Cannot be Used for Pre-Award Expenses
According to the Association of Fundraising Professionals' Ethics and 90%+ funders' instructions, "no part of grant funding can be used to pay pre-award expenses" (contracts made prior to paying fees after), grant writing fees cannot be paid from awards. Grants are not guaranteed so grant writers do the work and need to be paid for their work. Contractors have to be aware of this ethical issue with funders and with their businesses. Pay them for the work and hope and pray for the best to keep both of you in compliance.
As a side note, there is no standard fee for nonprofit work. You have done well to research costs but only use the amounts as "ballpark" figures with a range. Consultants are allowed to request "reasonable" fees for their work without being questioned about charges. Those fees are subjective. Keep doing your research and vet all your consultants/writers. More grace for much success.