Is a contractor going to get the best deals on materials?

Not necessarily. If you go to any lumber yard and tell them that you are going to spend a substantial amount of money at their place of business, you can usually come pretty close to the lowest price they would give anybody for the same order. An exception would be the big warehouse stores, because their prices are usually as low as they are willing to go, and quantity means little to them because they have so much business already. If you don't buy it, someone else will. A real lumber yard, however, will want the opportunity of supplying your project with the necessary materials to establish favorable local references and repeat business.  

Doing it yourself...