You can pick up the Lionel smoke unit for around $30 on the Lionel parts site. Obviously, you have to add the shipping of $10. The MTH PS/1 smoke unit goes for a bit more, it's been a long time since I bought one of those, but I seem to remember around $50. However, the lowly Seuthe costs more than $20, and you really need regulated power for it, so it's not exactly a bargain.
For a large building, I've seen a couple folks actually build a smoke unit, it's not a difficult task. A 40mm computer fan, some resistors, and a PCB are the primary ingredients. I was just looking at this issue the other day, and I started a PCB. I have to add the fan regulator and connection. The hole in the corner is the fill hole, and the hole on the opposite side of the fan hole is the smoke exhaust. This whole board is sealed on a project box about 1" deep. The wicks for the banks of smoke resistors hang down into the smoke fluid, it's filled to about 1/4" of the resistor bottom. I'd make the box from clear plastic so I could see the fill level.
Harry Henning made something similar for a large building, this is just a step-up from what he created.
Giant Smoke Unit PCB.png