Like so many things, “it depends.”
We are in East TN, west of Knoxville, and most of our place rolls up and down. Except for the mountain behind the house. The doesn’t “roll” up and down, it “juts” up and down!!! 
We also have some creek and river bottom flats that are lovely.
We have a covered 20m x 60m arena and an open 40m x 90m (more or less) arena. I don’t recall the specific costs on the outside but the covered turn-key costs were $64,500 in 1998.
The ground and planned use will decide the costs of any project. Our open was just a clearing and flattening exercise with some bordering drainage swales. No base of any kind and grass footing. My tenant, who trains and shows jumpers, rides on it regularly without injury to either her or her horses. After a heavy rain it’s out of service for a day or two. In our droughty time of year it gets pretty hard. Weather will often dictate usage pattern in open arenas no matter how well constructed.
We ride with a lady whose place was, literally, “carved out” of a hillside. The prior owners spent a boatload of money to do this. A friend who knows about such things estimated the rental cost of the equipment used (we drove by regularly during the construction) was $1000/day. But it drains beautifully. That’s what money can do!!!
The economics are that in mountainous terrain the bottoms will go for more money than the hills as they are more productive. They are also easier to build on. You don’t have to drill through as much dirt to sink a well. I’m sure you can think of other advantages. The foregoing presumes, of course, that you’re not building on a flood plain and will have drainage control for water coming off of the surrounding hills.
If you want to buy flat land then you have to go where the land is flat. Like IL, IN, or IA. If you are in the more hilly areas of VA then you won’t find what you will in the Midwest. So if you want a flat you’ll likely have to make it. That’s not a problem if you budget permits it. If it doesn’t then you have to consider other choices.
Good luck in your search.
G.