Three things:
1. Separation of building materials and crafting/whatever materials.
If building materials can only be used for building, more building will be done. This could backfire if you have one of those globalized, server-run shops that basically ruins everything already anyways. If the economy is player-driven, someone has to want it for building for it to be profitable, and if it's intertwined to some degree (e.g. trees can drop building wood or crafting wood and its random which one you'll get) with the crafting/whatever resources, then there will be inclination to build as everyone will have some. In retrospect I'm not entirely sure about this concept but I felt like saying it.
2. I think it's important to A) give the players reason to settle down and B) make those settlements be in danger and C) make building a viable method of protecting those settlements and D) if at all possible, encourage settling with other players.
One idea that comes to mind would be to make it so that settling has large advantages, but also requires a large investment. Settlements can occasionally be accosted by brick-damaging monsters, and if they destroy your original settlement brick (whatever that may be) or other valuables you've placed, you lose that investment and those advantages until you can afford to re-settle. From there, every settlement brick / other important bricks must be X distance from eachother, which forces a more spread out town rather than everything clumped together in a room that has huge thick walls. Suddenly building towns becomes viable and important. Its true that they could just surround the settlement brick with 1xXx5 bricks, but it wouldn't be easy to protect that; a series of walls around the township not only gives you something to defend from but slows down the monsters and makes them less condensed. If they all surrounded a settlement brick encased in x5 bricks, the structure would collapse quickly.
3. mfloor(%brick.getdatablock().getvolume()/4);
Should get about 50-100 wood / tree with that. You could set a max volume that a player could build to set up the other restrictions.
If you reduce it to /12, volume cost measurements are less precise, which may help remove the minimalist in all of us.