Finding the sum of the lowest two values from a table in MySQL
SELECT Price
    FROM Trading
    ORDER BY Price
    LIMIT 2;


will find the 2 lowest values.

Wrap that to get the SUM (or AVG or whatever):

SELECT SUM(Price) as sum_price
    FROM ( SELECT Price
            FROM Trading
            ORDER BY Price
            LIMIT 2 )


Then (I think) use that instead of the subquery you have.