i dont think anyone really knows, its pretty hard to tell the difference between 5% and 10% or 5% and 20% advantage even. i think people generally agree, it doesn't seem to be that big of a factor (i just assume its like 10% when the other school has it, figuring thats a fairly safe bet, and roughly discount it when i have it). the other problem is, favorite school comes into play so rarely in battles, because its not tied to the region (which is dumb), there really is not much evidence about it floating around.
given that you cant really rely on it for much, if anything, i would not let it be a big factor in your decision. sounds like you definitely would not pursue the recruit if he was not your favorite, so you probably shouldnt do it. however, it does present an opportunity to get a feel for the value of favorite school, which could be useful, so you might want to try it for that reason.