The cheapest of all BLGs on the market today (and readily available) is the GoPro BLG and Controller (Martinez v1) from RcTimer. Unfortunately that gimbal comes zero, zilch, nada instructions. Also there were some incorrectly machined parts. Hence building it can be quite a challenge. This Instructable is meant to fill this gap. Also it might give you enough pointers to design and build your own gimbal from scratch.
Testing of finished gimbal:
Timelapse video of the build:
Step 1: The parts
Step 2: Build the camera cage
Be sure that the grub screw holes both face forward. A GoPro should friction-fit in the frame pretty securely. Use Loctite on the screws or they might loosen and get lost quickly.
Step 3: Assemble the outside frame
We drilled a bevel using a larger drill bit. That fixed the problem nicely.
Step 4: Install pitch motor and bearings
We found a problem with the grub screw hole. It wasn't properly tapped, so we couldn't get the grub screw inserted all the way. After some debugging, we ended up switching the two sides, so the working grub screw was on the motor side.
The grub screw uses a 1.25mm hex wrench. It would be nice if RC Timer included one. They don't, so be sure and have one ready.