That's reasonable. I saw the potential expense, and assume $1-2ea used from an electronics recycler. One of my concerns is that PC fans aren't waterproof enough for outdoor use.
As for the blade design, maybe follow
www.youtube.com/c/MajorHardware for a 3D printable design.
The main point for me was a design that works in the wind shadows near the ground. Here's another video by Murray-Smith that addresses the same issue: