Appraisers go thru years of education and practice to become and stay certified.
While there is still some room for subjective differences in appraisals, they take their ethics very seriously.
You could ask a couple banks who do they use.
Especially Land Bank type banks will have appraisers that are doing that every day and will have comparables and the knowledge to look at everything to come with a fair price.
Those serious appraisals are not cheap.
Better agree up front who will pay for that.
Sounds like what you have there is an estimate, not a solid appraisal.
Since that estimate seems not quite right to you, maybe consider getting a real, complete appraisal.
Seems like a solid appraisal, the kind banks use for mortgages, would suit your situation best as a starting price point to discuss with the seller.