Here is the tweaked formula:

=SUM(MIN(CEILING(Purchase!B5,1000),Calculations!A30)*Calculations!B29,MAX(0,MIN(SUM(Calculations!A29:A31),CEILING(Purchase!B5,1000)-SUM(Calculations!A29:A30)))*Calculations!B30,MAX(0,MIN(SUM(Calculations!A29:A32),CEILING(Purchase!B5,1000)-SUM(Calculations!A29:A31)))*Calculations!B31,MAX(0,MIN(SUM(Calculations!A29:A33),CEILING(Purchase!B5,1000)-SUM(Calculations!A29:A32)))*Calculations!B32,MAX(0,MIN(SUM(Calculations!A29:A34),CEILING(Purchase!B5,1000)-SUM(Calculations!A29:A33)))*Calculations!B33,MAX(0,(CEILING(Purchase!B5,1000)-SUM(Calculations!A29:A34)))*Calculations!B34)/1000