Yes, buying at a guild store is faster. Here's the breakdown - most mats should cost $10 per unit. Ie, 200 iron ingots for 2000 gold. Your writ will require something like 30-45 units (iron cuirass, 15, pauldrons 12, sabotons 12 = 39 units = $390).

Lower level items are rarer / more expensive because people generally level and these items stop spawning (sort of...the issue is more that there are only a few players playing lvl 45 zones that drop thick leather, compared to a million of us v16 yahoos playing areas creating a glut of shadow hide.)

Your best bet is to work your crafting up and spend on the passives so you're getting writs for the materials that are common/cheap in guild stores. For example, shadow hide and void stone (level 10 writs, I think...next to last level) are way abundant and can be even cheaper than 10 gold each. And these higher level writs reward 500-600 gold each, so that you can still run a small profit even if buying everything from stores.

The other option is to "match" your writs to your character level. What the heck does that mean? As your character level increases, you'll get drops that correspond to that level. If you're level 6 and a sword drops in a dungeon, you'll get iron. That same dungeon will drop...um...orichalum, I think...when you're level 45. Check the charts. If your crafting passives match what you're picking up, so that you're being asked to craft orichalum gear while this is the drops you're getting, you'll likely have enough mats for writs from deconstructing and never have to spend on anything. This isn't as hard as it sounds. You'll notice all of a sudden what drops is a different wood/metal/leather, then go spend three skill points in the three lines amd your writs will all shift to the new mats.