What is the average cost for builder grade carpet and installation?

There really is no such thing as "builder grade " carpet. The closest definition would be low cost carpet. When buying carpet the general rule is the more it costs the better the carpet. The cost for installation is a bit trickier because installation prices can vary widely from area to area and also depends on what the installer will be doing ie; wood floor or concrete, a lot of seams, hiring a helper, etc. Unlike buying carpet paying more doesn't necessarily mean a better installation. So, the answer to your question in my opinion and in my area of the country would be about $9 per square yard for carpet and installation (not including padding). But at those prices I honestly wouldn't expect much.