i would apportion based on FMV.
if its 5 lots with the appraisal district, why don't you just use the appraisal value of each lot over the total multiplied by the purchase price?
in a perfect world you would get a real appraisal to get actual FMV and not a fictional value from the CAD, but im sure the CAD allocation would be close enough. and you're out the cost of an appraisal too under this approach.
1.167(a)-5 governs apportionment of basis in a depreciable property scenario and for business purposes. but this is close enough.
1.167(a)-5 Apportionment of basis.
In the case of the acquisition on or after March 1, 1913, of a combination of depreciable and nondepreciable property for a lump sum, as for example, buildings and land, the basis for depreciation cannot exceed an amount which bears the same proportion to the lump sum as the value of the depreciable property at the time of acquisition bears to the value of the entire property at that time.