... I don't know of any 'rule of thumb' because there are a lot of variables involved in home building, even when it comes to just pricing a foundation.
For example, is the soil sandy? If so, how deep is it? Where is the house going to be built? (In the mountains, on the coast, or in the rural lands of GA?) And so on.
I suggest you contact a builder that's built homes similiar to the one/type that you want. He, having already built ranchers, should be able to give you a ballpark figure. Again, there are variables he'll most likely mention to you. If, for example, the concrete for the foundation has to be driven 100 miles vs. 10 miles, that alone, affects the cost.
Before you do anything, sit down with the other part of the 'We are building a ranch in NW GA' and discuss 'how' you'd like your ranch built. (Concrete foundation vs. block foundation, above ground vs. below ground basement, 9' high unfinished basement vs. 3' crawlspace, and so on.) If you can do the rest of the house in a similiar way, I think that your ballpark estimate will be a good one. Otherwise, you're leaving it up to the builder/architect to 'write in' material that you may NOT want as well as 'home features', thus, affecting the cost. Sometimes, it's money well-spent to hire an architect to do prelims for you especially if you're building a custom house, and doing so for the first time. If you'd like a sample of this 'process', watch Dream House on HGTV. This show chronicals the process of building a dream house. Even though your ranch MAY not be your 'dream house', the process is worth watching (especially if you're not into reading about the process in a book. And believe me, there are a LOT of books on this subject!)
From there, perhaps others have some more info. My best to ya and hope this helps.