Is there any consensus about the relative merits of a second placing compared with a first on average?

I notice that prize money is allocated with a second receiving approx 30% of first prize whereas a third gets about 15% of first prize. Is there some rational for this allocation?

Taking a different approach, if on average there are 10 horses per race then a second is probably worth 90% of first.

I am only considering averages and realise that an individual result will depend on margins and other factors.

John