The cost per square foot to develop shouldn't change that much from a single family for a duplex... you just have to look at what the cost per square foot is and multiply it by how many square feet you're building. 

Can someone explain how the Gross Rent Multiplier works?

All items entered into the BRRRR Calculator as expenses (water/sewage, taxes, insurance) will automatically be multiplied times the Rehab period and transferred to the Rehab budget.  

@Joe Farruggio Multiply what it will be worth *.8 (80 percent).

At a minimum you'd probably be looking at 4% (multiply annual rent by 25).

Returns, cap rates and gross rent multipliers are several ways that people begin to look at markets.  

"The multiplier", meaning the 100 times I mentioned (which is often referred to as the 1% Rule) vary by which property we are talking about.

Determine this value by estimating the market rent the defining, then multiplying by the gross rent multiplier in your area.3rd option, nightly rental. 

Would anyone know what the general Gross Rent Multiplier (GRM) is for the Asheville area for multi-family properties?  

Relax, it's not going to be $300B, double that, add your shoe size and multiply times Pi.