Actually, it's not entirely certain how this works...
Try looking up the thread from a few weeks back on how to craft mithral armor.
Specifically, the way the game rules read is that, in order to make a Masterwork item, you first craft the item normally (which, for a greatsword, is Craft (Armorsmithing) check at a DC of 15).
Your progress is measured in Check Result * DC silver pieces per week, and you are finished whenever you reach the actual cost of the item - not the materials cost! (This is a mistake I made before.)
So, to make a normal greatsword, assuming a +12 Craft (Weaponsmith) bonus and taking 10, you'll make 22 * 15 = 330sp of progress per week. Since a greatsword costs 50gp, it'll take you a little less than two weeks to make a normal greatsword.
*Then* you craft the Masterwork component, which has its own cost and DC (150gp and DC of 20, respectively). After paying 50 additional gp for materials costs, you'll make 22 * 20 = 440sp of progress per week, meaning you'll finish off your masterwork greatsword in just under 5 weeks of work.
Now, the question is where does the extra money for the admantine part of the sword come in? For a sword, that's 3,000gp worth of progress you need to make, of which 150gp is the Masterwork component.
So, is the remaining 2,850gp worth of progress done as part of crafting the sword (in which case the progress is made at 22 * 15 = 330sp per week, taking a total of just over 91 weeks [just shy of two years] to make), or is it part of crafting the Masterwork component (in which case progress is made at 22 * 20 = 440sp per week, taking a total of just over 69.5 weeks to make)?
The rules do not specify in this case.
One thing is certain, however, it is nearly impossible to complete anything made of a special material unless you can easily succeed at a DC 10 higher than is required.
So, before starting on a fancy armor creation binge, you might want to find someone with the Craft Wonderous Item feat, and have them make you some Tongs of the Armorsmith - (+2 Circumstance bonus for being MW artisan's tools, +10 Competence bonus to Craft (Armorsmith) checks, for a rough price of
10,000gp).
In that case, you can reduce the time to just under 38 weeks using the first method, and just under 32 weeks using the second, assuming you don't add another skill point to your Craft skill.
EDIT: Sorry, extra 0 on the Competence bonus item.
