Queries :: Average IF Statement-syntax Error (missing Operator) In Query Expression

I am trying to get Average If function to access sql. I have columns Period and Costs_Per_Capita, result should be like like this:

Costs_Per_Capita Period CALCULATED_Period_Avg_Costs
15,505 1 15976.27582
16,368 1 15976.27582
16,037 1 15976.27582
15,995 1 15976.27582
15,000 2 16000
17,000 2 16000

I tried:

SELECT Costs.Costs_Per_Capita, Costs.Period
IIF (Period = 1, (Select AVG(Costs_Per_Capita) From Costs Where Period = 1),
(Select AVG(Costs_Per_Capita) From Costs Where Period = 2)
AS result
FROM Costs;

But get "syntax error (missing operator) in a query expression ..."