I work with DMC blanc (white) floss. Cones of various sizes can be purchased here and there, but I've found the best value for retail purchase is the 500-gram cone (1.1 lbs.). I can wind around 75 thirty-yard skeins for dyeing from a single cone.
I buy mine at JoAnn.com. It is useful to sign up for the JoAnn e-mail notices, for these frequently contain coupons and specials. I find the company will sometimes run very short-term special offers, such as free shipping, and the only way you will know about them is via the e-mail notice.
The usual retail price on the 500-gram cone is around $35. This past July fourth weekend the website had a short sale: free shipping on any purchase over $40, and cross stitch supplies were all 40% off. I bought two cones, and this will keep me supplied for months to come.