Once you reach the point where ilvl 55 items aren't worth it anymore, it gets very expensive... your leveling options are:
• ilvl 70 crafted items or dungeon drops (up to around 85 Desynth skill, then it gets too slow)
• ilvl 80 dungeon drops (nice success rate / skillup ratio)
• 3-4 meld ilvl 90 crafted items (don't bother before 85 Desynth skill, the success rate will be too low)

For Mastercraft Demimateria, so far I've only obtained them from crafting/gathering ilvl 70 gear. I've Desynthed about 40 Artisan's Spectacles so far. Obtained 5 Mastercraft Demimateria, some Rose Gold Nuggets and some Rose Gold Ingots. The Rose Gold Ingots can be re-crafted into Rose Gold Clasps, which are ilvl 70 desyth and have a chance for a Fieldcraft III Demimateria (3 so far), or materials such as Rose Gold Ingot again (repeat!), or Rose Gold/Darksteel Nuggets

I've never obtained Fieldcraft III or Clear III Demimateria from Artisan's Spectacles directly.