How to determine an ARV on a multimillion dollar house

[Jeet Sangha]

Hi all The seller thinks the house is worth 4.5M after repairs and selling it for 2.0M. House needs about a Million to finish. It's a newer home and hasn't been finished being built. The comps are few and far between since this house is huge 10k SF. The only way I feel like to determine the ARV is to get multiple appraisals. How would you guys go about putting an ARV on a house like this?