"Metal chelates are inherently more stable than closely related nonchelate complexes." This statement simply explains that the metal-polydentate ligand complexes are thermodynamically more stable than a metal-monodentate ligand complex (provided that the donor atoms are the same). I have seen several tables that shows this effect in a quantitative way like the one below: (It is adapted from the book I have just finished)
But, as explained in this paper (unfortunately, not free), chelate effect "decreases with the increase in concentration of the ligands and can become zero, or even negative, as observed in practice."
Since the article is not free, I think I am not able to copy and paste the equation derived to explain the statement above or the tables that show how chelate effect became negative with increasing concentration. I don't know much about this copyright thing, so I'll pass.
Anyway, I think it is important to know that concentration of the ligand has an effect on it. Until a few weeks ago, I did not know it. If you are interested in metal chelates, there is a FREE and detailed study of them by Arthur E. Martell here. I hope I will read it very soon.
 Dwyer, F.P.; Mellor, D.P. Chelating Agents and Metal Chelates. 1964. New York. Academic Press. pp 42