I use a kitchen/produce scale that reads to 10lbs for growing birds. Do the stand on a scale with bird in hand thing for grown birds. Of course the produce scale works exceedingly well for grown birds after they are butchered.
Exact scale I use from Walmart for under $15. I like the removable metal dish for ease of cleaning.
http://www.walmart.com/ip/Mainstays-Market-Scale-Black/35854697
This is cheap and works well enough for what you are needing. Used in the past for exactly this purpose!