Let the value of the highest prize be x. The sequence of the seven prizes will be:

$x, x - 20, x - 40, x - 60, x - 80, x - 100, x - 120$.

The sum of these prizes is given as Rs. 700, so:

$x + (x - 20) + (x - 40) + (x - 60) + (x - 80) + (x - 100) + (x - 120) = 700$.

Simplifying the equation:

$7x - (20 + 40 + 60 + 80 + 100 + 120) = 700$,

$7x - 420 = 700$.

Now, solve for x:

$7x = 700 + 420 = 1120$,

$x = 1120 / 7 = 160$.

So, the highest prize is Rs. 160. The least prize will be:

$160 - 120 = 40$.

Thus, the least value of the prize is:

Rs. 40.