I wondered why the topic was wandering somewhat but I see that it's built into the thread.

I had only seen the thread title, but see now that the thread title and the OP are asking two different questions:

Thread title: "How much Highland attire goes to the Highlands?"
OP question: "how much of the current market for Highland Attire is in Scotland as compared to the rest of the world?"

I had spoken to the question in the thread title, saying that we could only expect a small amount of merchandise to be purchased in the Highlands due to its very small population (the Highland Council site giving the population of around 230,000 or a little over 4% of the total Scottish population).

Now about Scotland as a whole, the kilt hire industry and the Army both purchase traditional kilts, sporrans, hose, headdress, etc in large quantities. Some large kilt hire concerns have several locations in various Scottish cities and I would guess that they might purchase kilts by the hundred. There are hundreds of pipe bands in Scotland and these purchase kilts, sporrans, hose, ghillies, jackets, etc on a regular basis.

For myself, I would limit the conversation to traditionally made Highland Dress. If you throw Utilikilts into the mix, they're made here on the West Coast of the USA and most are sold and worn here.

Scottish firms exporting Highland Dress overseas is nothing new! In my vintage Highland Dress catalogues from the 1920s and 1930s these Glasgow and Edinburgh firms point out repeatedly that a large percentage of their business is export.