I've built many custom fan-driven smoke units...not so much because of cost but because a stock smoke unit would not fit. When you think about it, a smoke unit has very inexpensive components. A $1 DC motor, a fan/impeller blade (which can be hand-crafted from thin sheet metal), a smoke heater resistor (I'd buy them for $1 from MTH), and a smoke chamber which I'd typically fabricate for the job-at-hand using thin brass sheet metal or brass tubing.
The key point is the electronics is the same whether driving a $50 MTH fan-driven unit or my <$5 (in parts) custom fan-driven smoke unit. If I were to summarize some take-aways, I'd say you don't need as much airflow as you'd think to stream smoke. And that the more interesting smoke effects are intermittent or burst-like such as simulating train puffing or startup exhaust from internal combustion engines or firing weapons. It's happenstance that the pulsing electronics that can start/stop inexpensive fan-based DC can motors works out for these applications.