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: