Step 1: Use the given information to find the side length of the square.
In a square, the diagonals are equal in length, and they bisect each other at right angles. Let \( s \) be the side length of the square. Using the Pythagorean theorem for the right triangle formed by two sides of the square and the diagonal, we have:
\[
{Diagonal}^2 = s^2 + s^2
\]
Since the length of the diagonal AC is given as 50 meters:
\[
50^2 = 2s^2 \quad \Rightarrow \quad 2500 = 2s^2 \quad \Rightarrow \quad s^2 = 1250
\]
\[
s = \sqrt{1250} = 35.36 \, {meters} \quad ({rounded to two decimal places}).
\]
Step 2: Find the area of the square.
The area \( A \) of the square is given by:
\[
A = s^2 = 1250 \, {square meters}.
\]
Step 3: Calculate the total cost of laying grass.
The cost of laying grass is Rs. 5 per square meter. So, the total cost is:
\[
{Total cost} = 1250 \times 5 = 6250 \, {Rs}.
\]
Step 4: Final answer.
The total cost for laying grass in the field ABCD is Rs. \boxed{6250.00}.