I'm not quite following how your description gives you the result, but I followed your written directions and got this:
A = [1 2 4 7; ...
2 3 1 6; ...
4 5 6 15]
% Sort last column in ascending order:
rightColumn = sort(A(:, end), 'ascend')
% Get the cumulative sums of that
c = cumsum(rightColumn);
% Append onto right edge of A as a new column.
A = [A, c]
