Residential Funding Modelling Test - Determining a Purchase Price HELP

Hi Guys,

Long time fan  of the forum and I am coming to you today with an urgent request to help understand  how to  get to a purchase price   for a modelling test practice.

I have the following information:

GIA - 50,000

Units: 54 

70%  OMV and 30%  Social Housing @ 35% of OMV

Demolition Costs: 5/sft

Construction Costs: 200/sft

I've been told  "You should target a geared profit on cost of 20% to dictate purchase price" and this is where I run into an issue. I've done a BTR model before know how to  determine a sale price based on a Cap Rate and NOI but I've not done a Build-to-Sell model and  had to work out a   Purchase Price.

Any help here?  No other information apart from in the next stage where  a 65% Debt Facility will be taken out. 

I'm just struggling to understand  how to get to the purchase price.