I love these rugs. You can purchase the one below at Sweet Creations for $299. If you wanted to try and DIY this, I did a tutorial once on how to make felted balls for Say Yes to Hoboken. They are fairly easy to make the only problem is that the cost of wool roving isn't cheap.
At 100cm in diameter, I can see why these are $299. It's become more prevalent since the time I wrote that article. Walmart even sells it! I used to only be able to find it on Japanese sites. But still the best deal for wool roving I've found is wholesale on Needleworks Pleasure. She quoted me for 22lbs of merino wool roving in 10 colors for $100.96 (including shipping to EU, still waiting on shipping quote to US).
If you want to figure out how many balls you'll need find the area of the circle. To find the area, square the radius (50x50=2,500) then multiply by π (3.14). Equals 7,850. Since each ball is 2cm divide that number by 2. So, you'll need approximately 3,925 balls. WOWSA! If one ounce makes 15 balls you'll need 261 ounces which FINALLY equals 16.31 pounds. Maybe less, that's if they were tightly packed.