Cost of fencing per metre = Rs 30

Total cost of fencing = Rs 12000

So, the length of fencing (perimeter) = Total cost/ Cost per metre

= 1200/30

= 400 m

Now, length of fencing = Perimeter of the square field

= 4 × side of the field

Therefore, 4 × side of the field = 400 m

or, side of the field = 400/4 m = 100 m

So, area of the field = 100 m × 100 m = 10000 sq m.