the logic of buying "costly" stocks

when u r sinking....where do u put ur dry cookies? 

in the container which has already sunk or in the one that is still dry? 

.....so, by this logic i keep buying/adding those stocks on every nifty decline which are not falling or have fallen much less....

earlier i used to buy the stock which was available much cheaper than earlier....the one which has fallen the most....instead of those which were still relatively dearer.....