You are correct. You can safely use considerably less heatsink material when actively cooled. Like I posted earlier, it’s about 6" squared of surface area for active cooling. I wanna say that the magic number was around 17" squared for passive cooling. And those numbers are per heat watt, so you shouldn’t have to calculate the entire power of your cobs. Also, I think that active number requires fan blowing directly on sink, keep that in mind while laying everything out.
The prices on the passive pin sinks does seem a little high sometimes. When considering the alternatives, it’s not ridiculous though. I’ve used the 5.886 extrusion from heatsink usa, it comes I’m around $2 per inch shipped. A decent 120mm fan is usually $10-15, power supply, wiring, and connectors add up too. When it’s all said and done, it’s not really any cheaper unless you have stuff to use already. Even then, any difference can be made up over time if you calculate the energy fan consumes into your system efficiency.
The only way to get over is be able to recycle sinks or use salvage parts. If you’re not having to buy any heatsink material, the efficiency loss from power supply for fan and fan itself is negligible. Probably 10-20 watts total per hour. As long as what you have is keeping the cobs cool enough to not hurt led efficiency, you’re talking about a couple dollars of energy cost per grow.