What are the SQL Server 2012 versions of FIRST() and LAST()?
You were close - FIRST and LAST are from Access; in SQL Server (starting with SQL Server 2012) they are FIRST_VALUE() and LAST_VALUE(). 

So, if you are 2012 or better (or Azure SQL Database), here's one way to get your answer:

CREATE TABLE #fl
(
  IdentityColumn INT IDENTITY, 
  Value INT
);

INSERT #fl(Value) SELECT 10;
INSERT #fl(Value) SELECT 45;
INSERT #fl(Value) SELECT 65;
INSERT #fl(Value) SELECT 95;
INSERT #fl(Value) SELECT 200;

SELECT TOP (1) LAST_VALUE(Value) OVER (ORDER BY IdentityColumn)
            - FIRST_VALUE(Value) OVER (ORDER BY IdentityColumn)
  FROM #fl
  ORDER BY IdentityColumn DESC;

GO
DROP TABLE #fl;