The special material price is just that: the price you pay for the special raw materials. That is, if you're making mithral armor, you first need to spend money to buy some mithral ingots. That can be a large percentage of the final item's market price right there, but it doesn't use up Crafting time; you could purchase wagonloads of mithral without ever setting up a forge.
Full plate is heavy armor, so the raw materials include 9000 gp worth of mithral. But crafting something out of mithral takes only as much extra effort as crafting a masterwork item-- that is, 300 gp worth of effort at DC 20. That's what the DMG means when it says mithral items are "treated as masterwork" for purposes of creation time.
So, for that blacksmith with his six apprentices:
19 weeks for the armor itself
+ 3 weeks for the masterwork (special material) component
= 22 weeks, or about 5 months.
Jeremy, your calculation only makes sense if the blacksmith and his apprentices are also using "Craft (mining)" to find, extract, and refine the mithral by themselves. In such a case, 2.5 years may not be totally unreasonable, because you need more than 7 people to run an ore mine efficiently.