[Страдалецъ]

Можно и без промежуточных телодвижений:

Код:

SELECT * INTO Table3
FROM (SELECT Tmp1.DateField, Sum(Tmp1.PriceField) AS SumOfPriceField
             FROM (SELECT DateField,NumberField,PriceField 
                          FROM Table1
                          UNION
                          SELECT DateField,NumberField,PriceField 
                          FROM Table2)  AS Tmp1
GROUP BY Tmp1.DateField)