I'm not a structual engineer, but I'd like to help. I've worked on homes, and with different contractors over the years. It's a good idea to talk to a structural engineer, if the damage is bad. I've read this might run you around $500. They can determine the cause of the problem. They could test the soil under the foundation to see if uneven soil is causing the foundation to settle. Clay in the soil is another frequent cause of foundation settling. To test the Soil, it could cost $500 to $1500. A less expensive option might be to contact a local university, local government official, or a soil engineer for topographical maps of the area. These could tell you what kind of soil the house is on. Maybe try hiring a geology student , who could help you for less money. Non major foundation cracks could cost around $500 to $1,000 a piece to repair, which involves cleaning out the crumbling concrete and inserting caulking. Normally there is more than one crack, like you discovered. Slabjacking, also called mudjacking is a process of pumping a mix of cement, water and sand under the existing concrete, to lift the uneven foundation back into alignment. In some areas the average price for mudjacking is approximately $1,600 to $3,600 depending on how much work is involved.
Since the house is small you need to be careful not to overspend on repairs or you may not get you money out of it. Citrus Heights is a nice city, but does not demand the house prices like Rocklin, Folsom, or some of the other local cities. Always consider "resale" when working on a house.