How to merge 2 data sets?

Start off by creating a new table which has all the codes from both tables. From the Modelling tab in Power BI Desktop choose New Table and enter

All Codes =
DISTINCT ( UNION ( DISTINCT ( 'Table1'[Code] ), DISTINCT ( 'Table2'[Code] ) ) )

Now, in the Model view delete any relationship which might have automatically been created between Table1 and Table2. Create one-to-many relationships from the All Codes table to both Table1 and Table2.

Add 2 new columns to the All Codes table,

Exists in Table1 =
NOT ISEMPTY ( RELATEDTABLE ( 'Table1' ) )

Exists in Table2 =
NOT ISEMPTY ( RELATEDTABLE ( 'Table2' ) )

Now you can create a measure like

Price difference =
AVERAGEX (
    FILTER (
        'All codes',
        'All codes'[Exists in Table1] && 'All codes'[Exists in Table2]
    ),
    VAR Table1Value =
        SUMX ( RELATEDTABLE ( 'Table1' ), 'Table1'[Price] )
    VAR Table2Value =
        SUMX ( RELATEDTABLE ( 'Table2' ), 'Table2'[Price] )
    RETURN
        Table1Value - Table2Value
)

Finally you can create table visuals with the Code column from All Codes and use filters to show those codes which exist in one table but not the other and a visual with the code and the measure showing price difference.