The outside walls of my entire barn are made of pallets. I nailed them to 4x4 posts then lined the inside with wood and slapped the tin siding on the outside. Even the stall gates are made of pallets. I took them apart and removed the spacers and nailed them back together. That is how I was able to build a 6 stall barn for less than $400. It is all made of re-cycled materials, the only thing I had to buy was the hardware (nails, hinges, stall latches, etc.). I am getting ready to double the size of my barn and will use the same thing, pallets are usually free and by using them for the outside walls, insulation will slide right in the space made for the pallet jack.
I tore down a couple of old barns and was very careful removing the tin and I took them apart so I was able to save the wood and the tin. Total cost of the barns $0 just a lot of time and energy.
If I wasn't such a good scrounger, I wouldn't be able to have 1/2 the things that I have.
Mary