SQL view, better way?
First off, it's not a stored procedure, it's a view.

That said, the main change I would make is to use a CTE, once you have upgraded.

ALTER view [dbo].[JobCost_EstVsAct]
    --SELECT * FROM JobCost_EstVsAct Where jobnumber = '122773'
AS
WITH cte1 as (
    SELECT JobNumber, CostCenter, Amount, 0 as cost
    FROM Avanti_ActiveJobBudgets AJB
),
cte2 as (
    SELECT jobnumber, costcentre as CostCenter, 0 as Amount, cost as Act
    FROM Avanti_ActiveCostDetails
),
cte3 as (
    Select * From cte1
    Union
    Select * From cte2
)
SELECT JobNumber, CostCenter, sum(Amount) as Est, sum(cost) as Act
FROM cte3
--where cost + Amount &gt; 0     /* This line is a bug */
GROUP by JobNumber, CostCenter
HAVING sum(Amount) + sum(cost) &gt; 0   --bug correction


Replacing the cteX with more appropriate names.