It contains a grand total of three components, the only one costs anything is the SD slot. At 100 units, they're about 50c each. The other components cost nothing, relatively, you can buy a thousand resistors for a dollar. The board itself would cost under a dollar to make at anything above 10 units. Total cost is about $2 if we're stretching.

I ran a quote for an assembly house, and for 100 boards it would cost around $200 or $2 a board - so now it's $4. Standard markup is 3 times the build cost or about $12-15. Yet it costs $25. That doesn't include international shipping, by the way.

Economy of scale certainly works when you're making 100.

Even Seeed Studio do assembly nowadays, and you don't even need to carry the stock as the components are in their Open Part Library.