If cost is the only consideration for the foundation for your home, clearly a concrete slab is less costly to pour than excavating and, in essence, adding an additional floor to your home. But don't stop thinking about cost at simply the construction stage. Take a look around at other homes in the area. If they have basements and you decide on building on a slab, the value of your home will likely be less should you decide to sell. The up-front cost may be less, but you will likely get a more comparable (higher) price at sale time. Perhaps enough to warrant the up-front costs. There are other considerations that, over time, will affect the cost of home building, as well.
If you are building on land with a high water table, the cost of a basement just became more expensive. There are several reasons for this. The most obvious is that water will be wanting to get into your basement all the time. Not an insurmountable problem but one that requires the addition of exterior french drains to allow water to flow from around the basement, water sealing the floor and walls, the possibility of adding a sump pump and the likelihood of higher humidity in your basement and home than if you simply built on a slab.
What's the Soil Like?
If the ground is filled with large boulders, add the excavation expense of removing the boulders to the preparation to pouring a concrete basement. Consider also the cost of the land. Where land is more expensive, the temptation is to build on a smaller and cheaper footprint rather than spread out the square-footage of your home over greater and more expensive acreage. Then there is the soil itself to consider. All houses settle, but the problems occur when the soil settles at different rates under a foundation, either slab or poured basement. Fortunately, the load of homes is considered relatively light, but a quick trip to your local U.S. Department of Agriculture will provide you topographic maps that will indicate the types of soil that underlie the home. Also, talk to local home builders about their knowledge and experience with the soils in the area.
In most earthquake prone areas, California for example, most homes are built on slabs. The geography and likelihood of land movement make it problematic if your home is in an earthquake-prone area and the basement is damaged. If it is built on a slab, the remedies are not only easier to fix, they are less expensive. The trend remains that homes in the West, unless on slopes, are largely built on slab while in the Northeast homeowners prefer the added space of a basement.
Wiring and Plumbing
Without a basement, you'll need to find places to put your water heater, washer and dryer, and wiring and plumbing must go through the walls and ceiling. With a basement, obviously, you still need piping and wiring in the walls, but it adds greater flexibility for such things as water heaters and other appliances. It's both a space consideration and a resale consideration. Also, if you decide to add such amenities as home theater systems on the first floor at any future time, you can always tuck away the wiring in the basement ceiling. It's more a convenience than a cost.
Whether you build on a slab or foot the additional up-front cost of a poured concrete basement depends on a number of factors beyond the soils and geography. Finances play a big part in determining which direction you may want to go in addition to all the other considerations. The per-foot costs obviously will be greater with a basement than if you build on a slab.