This is only very slightly related to your question, but I have a recent experience at work that might shed some light on the question of flex circuits being spendy.
We have a device at work that has a sensor that moves back and forth with a little stepper motor. There is a 10 pin flat ‘flexible’ cable (NOT a flex circuit like you’re thinking of, but a real cable that has thick copper wires embedded in plastic) that goes from the mainboard to the sensor. We originally used off the shelf cables from Molex because that’s what the sensor manufacturer recommended. They cost about $1.80 each in low quantities.
Turns out the cable only had a rated life of like 5000 flexes. We saw a ton of cracked/broken cables. So we had to go to a true flex circuit. I was totally worried about the cost (because we are in such low volume). Molex has a division that does custom flex cables, and they wouldn’t even talk to us because our volumes were so low.
We found a small flex circuit vendor who created exactly the cable we needed, and even added a special type of rigid backing on one end (very non-standard). The new cables work perfectly, and are rated for millions of flexes. And the best part? The cost. They built me a single panel of prototype cables (50 or so) for something around $250. When we approved the design, they just sell us the individual cables at about $2.10 each, as long as we buy at least 100 cables at a time. I couldn’t believe that a completely custom solution, with special dies to cut the cables out, etc. cost only slightly more than the crappy off the shelf version. And the NRE cost was only $250!
We could have easily made these cables see-through (clear) I believe - the way they are now they are translucent with a slight brown color to them. I’m sure you can have your circuit custom made by a flex circuit company - shop around. You can get a good price I think. But you’ll probably have to buy a whole panel’s worth.
The tricky part would be soldering a big part like a BGA to something that’s as flexible as a flex board. However, I’ve seen complete 16-bit microcontrollers (100 pin) soldered directly to a flex circuit in a digital camera, so it can be done.
*Brian