The only company I am familiar with that uses calcined clay uses it as one of several choice additives to be custom blended in and not a primary material. Your percentage would be flipped and likely closer to 90/10 or less of the additive. Due to amount needed, cost of material, weight and transportation I suspect it is not used as the primary portion of mixes for arenas and his application is unique. I also suspect it can be imported from China cheaper than it is produced here in the states but still not a cheap product to make or ship.
I'm not sure where my guy gets it, but I don't think cost is a big consideration. He charges $21/ton for pure washed sand, $25/ton for pure calcined clay, and $30/ton for an "arena mix" that's 80% clay + 20% sand. I suspect it's the labor to mix it that adds to the cost. I'm getting the mix for $25/ton as a "friends & family" discount, and because it's still a relatively new product for him and he wants to experiment a little with somebody who can give him detailed feedback.
Just like with adding sands to clay where you need to use a high enough percentage to allow for bridging I would suspect the same holds true here. Bridging would be where the sand particles actually touch to allow for space for the water to move through more quickly as well being held.
My usage will be in two main areas: the enclosed corral where the horses will sometimes be detained, and the "foyer" (for lack of a better term) that leads from the corral to both of the pastures, and gets heavy horse traffic. The drainage swale that drains the corral & nearby ground runs through the middle of this foyer. We're thinking we'll go with an 80/20 clay/sand mix (his standard arena mix) inside the corral, because stable, non-mucky footing is our primary concern in there. Outside in the foyer, we'll go with a 50/50 clay/sand mix so that the water can drain more easily through the swale, but the footing still stays good as the horses tromp through that swale and climb a slight hill en route to the pastures.
Another friend recently expanded his outdoor arena. His original portion has pure sand, while the new portion is 80/20 clay/sand. He says that the pure sand is noticeably more slippery because the particles don't lock together. After it dries out after a rain, the pure sand also becomes rock hard until it's harrowed, but the clay/sand mix is still very usable. He'll be mixing the surfaces together as time allows, of course.
We've had a dry week here, so I was able to flatten out the dry hoofprints in my corral tonight. The sand should arrive Thursday or Friday. We'll see how it all works!