Flash memory is actually very cheap when you compare it with other memories made of silicon. In term of density (bit/nm^2), there is nothing better today: a flash cell is approx 3 times smaller than a DRAM cell. Additionally, today, a single flash cell can store up to 3 bits (Multi Level Cell, MLC as opposed to Single Level Cell, SLC). However MLC-based flash memories require more time to read and to program pages and have a lower life time.

As the price of a chip roughly depends on its die area, it also makes flash the cheapest memory existing today. Today MLC Flash is ~11x denser than DRAM, while SLC is around 3x denser, that makes it approximately 11 times and 3 times cheaper than DRAM, respectively. Look at the prices of DRAM and Flash in your favorite store, you should get close to these ratios.

In the future prices should lower as we make flash even denser. However there is a limit to this shrinking: in 3-4 years the price reduction of flash should slow down. Another issue I heard about is that if everybody in the world wanted to replace their magnetic disk to SSDs today, there simply would not be enough silicon available...