Since this is what I am aiming for, I thought I would post a bit of information about it.
It is currently 22.5m, but seems to be dropping. In the last month, it has dropped almost 10%. I believe this will continue since the daily & 5 day averages are consistently below the monthly average, indicating people are buying and selling for less than the market price. In my opinion, the lowest the shield will go is 15m, if that.
As you can see, the dragonfire shield has much better stats than my current dragon square shield, including an impressive +7 strength bonus, to hit a lot harder. Of course, these stats are for the charged version, which can be easily achieved after killing a few dragons. I will rarely use the charges, except when I am low on prayer, and have just a couple of monsters left.
D-fire shield only loses out on mage and range attack bonuses, but these are not important at all.