We have,

Length of fence covered = Perimeter of the square park

We know that,

Perimeter of square = 4 × Side of square park

= 4 × 250 m

= 1000 m

It is given that,

Cost of fencing of a square park = Rs. 20

Therefore,

Cost of fencing 1000 m of square park = 1000 × 20

= Rs. 20,000