Yellow brass is bringing about $1.60/pound at the scrap yard in Dothan, AL. I suspect you can get all you want for around $2/pound if you look some more.
For copper, contact plumbers and ask about their copper scraps.
Now, yellow brass (and red brass) is a pain to cast. It is dirty and looses it's zinc the longer it stays melted. For the prices you are quoting you can get silicon bronze which pours really nice and is what is used for statues and things.
Regards, Ray in FLA