I've been cooking sous vide for 7-8 years and have always used Ziploc (or another brand) bags to cook in because they work well and I am cheap. Most opinion I've read online has been that, cooking SV in them is not a good idea. I even contacted the manufacturer and although they equivocated quite a bit, the bottom line was that they did not recommend their bags for this use but that might just be a liability/insurance issue. Now I see on the Modernist Cuisine website that they are safe. Modernist Cuisine is a very well respected source for sous vide and other cutting edge cooking information. Here's the cogent paragraph from the attached link:
"According to the latest research, the safest plastics for use with food are high-density polyethylene, low-density polyethylene, and polypropylene. Virtually all sous vide bags are made from these plastics, as are most brand-name food storage bags and plastic wraps such as Saran wrap. Polyethylene is widely used in containers for biology and chemistry labs, and it has been studied extensively. It is safe."
Like I said, I use them even know I thought there might be a safety issue so I posted this for people who are paying a ton for SV bags. They might consider this cheaper option.