Most calls for this come from people building a new house, adding on, or putting up a shop. Once the walls go up, you won't see the foundation again, and it's the hardest part of the house to fix later. That's why I spend more time on the ground under it than on the surface, and why the design is settled before anybody digs.
The price moves with the size and shape of the footprint and with what your soil report calls for. Deeper beams, more steel, a post-tensioned design, or bad dirt that has to be dug out and replaced all add to it. A tight lot that the trucks can't easily reach adds to it too. Your written estimate puts each of those on its own line.
The schedule turns first on the soil report, the design and the city's permit review, which all happen before we dig. Pour day then depends on the weather. After the pour comes curing, the wait while the concrete gains strength, and we keep the slab damp through it so it doesn't dry out too fast before framing loads go on.