Actually the blower was free. I got it from the AC guys like 40 years ago. They have lots of takeaway parts they remove from old systems. Think mine was from a small gas powered heating system.
The trick is to find a small blower. Ones from like 3 ton units are way too powerful.
Sometimes you can find a squirrel cage from some other app.
A busted blow up yard thingy from someones Halloween setup might source a good blower.
Another source (in dry climates) is an old swamp cooler blower. One of the window or small sized roof units is just perfect for this.
Haunt some garage sales to see what you can find.
The $60 is for a sheet of plywood, 2x 4” thick MERV 14 (12?) main filters and 2x 1” thick MERV 5 (8?) pre-filters, some casters to roll it around on, and a switch and cord for power (from the junk pile).
Cleaning the pre-filters every day yielded about 1/2-3/4 cup of drywall dust. The pre-filters really helps keeping the main filters from getting clogged quickly.
The design is just a box with open sides to fit the filters on opposite sides. Then the blower mounted so it can suck air through the filters. It normally sits so the air exits the top. So the clean air goes up to the ceiling and the “dirty air” can flow down the walls and across the floor to the filter inputs.
I also put some plastic feet on one side so the blower can point horizontally and blow clean air into an enclosed area - like a small room or closet if someone was working in there. Wired up a two speed switch.
The thing is really quiet. That’s the trick into getting people to use it. If it’s howling away, they can’t hear the job-site radios, which is (as far as I can tell) an OSHA requirement.
Good luck in your hunt,
All the Best,
Jeff