I was lucky ... I found a store that was going out of business and I was able to purchase a large amount of Kona solids for $3.00 or less (If I finished the bolt, and I often did, the first yard was $1.50). I was able to purchase about a dozen different colors, at least 2 yards of each, more if I finished the bolt (I looked for skinny bolts!).
So I would say keep your eyes open for sales, even if it's only a few colors on sale, and when you see the sales - stock as much as you can.