1. Printed Terrain; My favourites for printable terrain are Stoelzel's Structures and World Works Games. Print onto paper, glue to foam board and cut and construct. Each building takes me 8 hours or so. For this reason I have created 2 and a half.
2. Plasticville Buildings; A lot of cheap plastic "O Scale" railroad buildings will work, but Plasticville works especially well because it is slightly underscaled. It was intended to be 1:64 with 1:48 windows and doors (according to Wikipedia at least). I find that even small buildings can look overly large on the table. Either way, I love these buildings, and they may be my new favourite way to create modern buildings. On that note, there is a new Fallout miniatures game coming I may play and be creating scenery for. My new "go to" is Plasticville. If you buy used lots on Ebay they are very cheap. I just purchased 7 houses, 1 garage, 1 shed, 3 stores, 1 garage and an apartment building for about $82. If you want interiors (which I do), plasticard is your best option. They do of course need to be painted. I am toying with the ideas of using printable interiors on cardstock. Still working on it. At this point this seems like the best option for cheap, labour free buildings. You can get bulk amounts of furniture on eBay for fairly cheap too. I estimate that for 30 to 40 bucks I will have all of my buildings furnished. At the bottom of this section you can see some of the furniture I'm talking about.
3. 28mm Building Manufacturers; Sarissa, and I'm sure dozens of others. They vary in price but work very well too. These take a lot to create overall, but usually look awesome. For price and effort though, I tend not to get these ones very often.
