Greetings Piyush,
Generally, the planned cost for a refurbishment order will be calculated in a similar manner to any PM Order. The difference is that you have the "damaged" and "refurbished" Material, which can be at different values (via split valuation):
Value of "damaged" material + Internal labour cost from Operation activities + External cost from external Operations + Cost of any additional spare materials consumed - Value of "refurbished" material.
Therefore, if the value of refurbished material is high, you might get a negative planned cost.
At Refurbishment Order settlement, the cost is settled to adjust the value of the "refurbished" material stock.