I think we actually had some figures on here previously about this and the strike rate for favourites went something like this:

Race 1 32%
Race 2 31%
Race 3 31%
Race 4 30%
Race 5 30%
Race 6 29%
Race 7 29%
Race 8 28%

But the price of winning favs did also increase progressively from R1 to R8 to compensate for the smaller strike rate.