Admittedly the number of high quality power supplies below $60 is not that great (especially above 500W), but there are some which can be safely used. For the following suggestions I am not really taking upgradeability into account, altho I have no doubt that they will safely power the build you have outlined so far:
Antec Neo Eco 520W 80Plus $60 ($6 shipping)http://www.newegg.com/Product/Product.aspx?Item=N82E16817371030
Antec Earthwatts Green EA-500D 80Plus Bronze $60http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035
These two don't come with power cords so you would have to reuse one you already have or get another one from elsewhere.
Antec High Current Gamer HCG-520 80Plus Bronze $60http://www.newegg.com/Product/Product.aspx?Item=N82E16817371047
That Corsair GS600 isn't the best thing you can get for $100, as you can get things like the following which are cheaper and superior:
Antec High Current Gamer HCG-620 80Plus Bronze $85 ($15 rebate)http://www.newegg.com/Product/Product.aspx?Item=N82E16817371048
XFX 650W XXX Edition (Grey fan) 80Plus Bronze Modular $89 ($20 rebate, $4 shipping)http://www.newegg.com/Product/Product.aspx?Item=N82E16817207007