low cost, low performance flexible PCB substrates

[ehughes]

Hello!

I have a prototype built on on a standard PCB substrate (0.4mm   .015").  It is a large design (10"x15") and has no active parts.    I am looking for alternate substrates that are lower cost than fiberglass that are flexible.    A finished thickness of .010" to .015" would be fine.

1.)  I need circuit traces on both sides.     One side is a large plane.       I am avoided silver printed ink as the resistivity is too high.  1/2oz or 1/4oz copper would be OK.
2.)   I am aware of standard flex materials such as Kapton, etc.    I am looking for something that is lower cost.   
3.)   I do not need a high performance material.   In some cases the part will be throw away.
4.)  I can get standard FR-4 in in the 10"x15" size for about $5.  I would like to push towards $1.50 in QTY 10k.

I have used LED strips that are on a flexible material:

https://www.adafruit.com/product/1138?length=1

I have acquired these strips at a very low cost from AliExpress.     I don't think the material is a fancy polymer such as Kapton.   I would love to identify what exactly they are using.   

Thanks!