If you have streaming stock prices which are automatically updated, I don't know if Data Validation will work. Data Validation typically works for values being manually entered into a specific cell, not by formulas or links.
You may need to look at something a bit more complex, like some Event Procedure VBA code (specifically, the Worksheet_Calculate event).
These kind of questions have been asked a lot on this board (streaming stock prices). If you do some searches on this board, you should be able to find many posts on this subject.