Create Table #myTable 
(
	ID char(2),
	Customer varchar(20),
	Pcs int,
	Wgt int,
	Cost decimal(15,2),
	ProDate Datetime
)

Insert Into #myTable
Select 'PS', 'Pollak', 25, 1001, 311.25, '1/20/2003' Union All
Select 'PO', 'Stone', 25, 650, 610.16, '1/20/2004' Union All
Select 'PS', 'Pollak', 25, 781, 898.77, '1/20/2005' Union All
Select 'PO', 'Stone', 25, 512, 199.68, '1/20/2003' Union All
Select 'PS', 'Pollak', 25, 956, 389.39, '1/20/2005' Union All
Select 'PO', 'Stone', 25, 684, 500.00, '1/20/2005'


Select * From #myTable

Select 
	id,
	Customer, 
	Cost2003,
	Cost2004,
	Cost2005,
	Proj2006 = Cost2005 *
			(case when cost2003<=0.0 then .5 else cost2004/cost2003/2 end
			+ case when cost2004<=0.0 then .5 else cost2005/cost2004/2 end)
From
	(
	Select 
		Id, 
		Customer,
		Cost2003 = sum(case when year(proDate)=2003 then cost else 0 end),
		Cost2004 = sum(case when year(proDate)=2004 then cost else 0 end),
		Cost2005 = sum(case when year(proDate)=2005 then cost else 0 end)
	From #myTable
	Group by id, customer
	) A

Drop Table #myTable