Slow SQL Server Query
Why all the ( ) and long names?
Why mixed lowerecase camel case on table names?    

Just basic cleanup

  SELECT dbo.currentadwordsmaxbid.maxcpc - dervtable.Bid AS Expr1,
         dbo.currentadwordsmaxbid.webid,
         dbo.currentadwordsmaxbid.maxcpc,
         dbo.currentadwordsmaxbid.adgroupid,
         dbo.currentadwordsmaxbid.keywordid,
         dbo.currentadwordsmaxbid.campaginid,
         dervtable.SKU,
         dervtable.Bid
  FROM   dbo.currentadwordsmaxbid
         LEFT OUTER JOIN dbo.WebSkuLookup
                      ON dbo.currentadwordsmaxbid.webid = dbo.WebSkuLookup.webid
         LEFT OUTER JOIN (SELECT SKU, campaginid, Bid
                          FROM   dbo.BidHistory AS ph1
                          WHERE  DTStamp = ( SELECT MAX(DTStamp) AS Expr1
                                             FROM   dbo.BidHistory AS ph2
                                             WHERE  ph1.campaginid = ph2.campaginid
                                             AND    ph1.SKU = ph2.SKU )
                        ) AS dervtable
                      ON dbo.WebSkuLookup.sku = dervtable.SKU
                     AND dbo.currentadwordsmaxbid.campaginid = dervtable.campaginid
                     AND dbo.currentadwordsmaxbid.maxcpc &lt;&gt; dervtable.Bid


row_number() 

  SELECT dbo.currentadwordsmaxbid.maxcpc - dervtable.Bid AS Expr1,
         dbo.currentadwordsmaxbid.webid,
         dbo.currentadwordsmaxbid.maxcpc,
         dbo.currentadwordsmaxbid.adgroupid,
         dbo.currentadwordsmaxbid.keywordid,
         dbo.currentadwordsmaxbid.campaginid,
         dervtable.SKU,
         dervtable.Bid
  FROM   dbo.currentadwordsmaxbid
         LEFT OUTER JOIN dbo.WebSkuLookup
                      ON dbo.currentadwordsmaxbid.webid = dbo.WebSkuLookup.webid
         LEFT OUTER JOIN (SELECT SKU, campaginid, Bid 
                               , row_number over (partition by SKU, campaginid order by DTStamp desc) as rn
                          FROM   dbo.BidHistory AS ph1
                        ) AS dervtable
                      ON dbo.WebSkuLookup.sku = dervtable.SKU 
                     AND dervtable.rn = 1
                     AND dbo.currentadwordsmaxbid.campaginid = dervtable.campaginid
                     AND dbo.currentadwordsmaxbid.maxcpc &lt;&gt; dervtable.Bid