I worked for a huge publisher for 7 years and many of their print runs (school textbooks) are around 4000-7000. I've seen how they work out their figures before a project is approved. If you don't think you're going to sell 10,000 copies, there's no point in printing that many even if the unit cost is lower. You'd end up paying for thousands of books you'll never sell (books that cost money to warehouse) and your overall profit on the project is lower.