The 1991-P Kennedy Half Dollar has one of the lower mintages in the Kennedy Half Dollar series, with the exception of several coins struck after 1998 as mintages were significantly lower for some issues after this year.
The 1991-P Kennedy Half Dollars were struck decently and examples up to about MS65 can be found easily. In MS66 they are much more difficult but they are somewhat common that they can be purchased inexpensively either certified or raw. In MS67 condition they are very difficult to find and worth a significant premium. Anything grading higher than MS67 is a very rare coin, as few examples if any are known in this condition.