The problem with this comparison(or situation) is that it's moot.
Unless you're willing to test the textile strength of the front element of the lens, there is no use in saying that the filter had protected your lenses front element other than that it took the brunt of the force of the stone.
There is also the situation that had the filter not been there that the lens front element may have simply brushed off the stone without any damage at all, and if that had been the case you could have saved yourself the $80.
The front element of the majority of lenses are much more resilient than a 0.5mm thin wafer of glass is(there may be lenses with less hardy front lens elements, but I've never seen or heard of them).
A while back, I had an old 'broken' lens that finally went to god, but before it did, I tried a few tricks with it in that I used a normal household kitchen scourer on the front element, initially with not much force and gradually with more force, where I ended up probably using as much force as I would with my burnt pots and pans(well.. I'm not a very good cook, but I'm very adept at washing up!
)
.. anyhow, not a mark on the lens front element, not a scratch, nothing. I certainly scratched the plastic housing around the glass element and it was easily very obvious that I'd taken to it with a scourer, but the glass itself was completely unmarked.
My opinion is that if the filter was only 'chipped' by flying debris, then the force of the flying debris was probably not high enough to cause damage to the lens element either.