The designers you named, Tula, Kaffe, Lori Holt... are all very popular right now. Their popularity adds to the cost of their fabric. (Supply and demand.) Those are fabrics you rarely see on sale, so you have to be prepared to shell out $12 or so per yard if you want them.
Kaffe is a phenomenon. His are the only prints I see being reprinted for years, while everyone else's prints get one shot and are then retired. I guess there is enough demand for his stuff that it continues to sell in subsequent years at full price.