I've built functional things, such as a frame for the bed of my truck, a workbench, etc. out of 2X4's. I don't use treated lumber, usually. Probably should have with the truck bed frame. Anyways, it wasn't so much that the wood was expensive (I can buy a 2X4X96" for $1.67 here), but the hardware - I think I paid a dollar a bold the last time I put it together. For me, that is where the expense lies.

When I'm building furniture or other household goods, I usually use oak. In part because I don't really know 100% what the finished product is going to look like. I have used black walnut, I have also used birch. For carving, I actually prefer oak. That gets expensive, but I usually don't have tons of that stuff around, either. AND I'm not building a fence out of the stuff.

Another option I've been enjoying since moving back to the south are going around collecting fallen wood, trees in the aftermath of a storm. I picked up some nice logs after a storm took down a few big trees in the past month or so. I realize I need to let that stuff dry out for a while, but am not really sure how long. They are really nice blocks, and I know I want to carve a bowl out of one, but am kind of at a loss as to where to go from here.