How to Estimate Renovations for Mansions

[Marcel Pean]

Hi,

What is a good estimate/rule of thumb to use to estimate a major rehab, full gut job for a large home? This house is in a very upscale neighborhood in Washington DC and has 5 bed, 4.5 baths. Homes in the vicinity sell for $1M+. In order to compete with the surrounding market, a rehabber would use more expensive materials increasing the value of the house. 

Has anyone even wholesale a house this big?

I appreciate any suggestions and advice in advance!