Your contractor may have enough experience in your market to make this kind of generalization, but when you get to the point of drawing up a CONTRACT I'd be very clear to specify the materials being used. You'll need to familiarize yourself with prices in order to do that. Just no escaping it.
You might find it helpful to look at 'The book on estimating' from BP, looking things up on Homewyse.com, and just wandering around Home Depot to get a range of prices. It might be helpful to decide on the type of property - A, B, or C that you're planning to buy beforehand as well. You don't have the freedom to decide on the grade of materials you use - the neighborhood you buy in and property class will dictate that. Estimating costs is just one of the reasons to decide the type of property you're interested in early on.
It sounds like your contractor may have been doing this for a while so you're lucky, but it's probably in your best interest to start learning this for yourself, too, today.
Best of luck.