Is this crack only on this side of the house? It looks like this wide crack (from the picture looks like is a good inch on one side then less on the other plus a vertical crack that seems to follow a brick wall against concrete slab foundation.
Is this house have a basement? I'm pretty sure that the slab is not resting along on a brick wall and must have some sort of retaining wall or columns/beam system. It might be just the separation from the brick wall and the concrete itself. I suspect water is a problem like @Roy N. said.
If there are some differential settlement (the house is sinking in a non uniform way), you should be able to see diagonal cracks on the bricks, specially from the corners of the openings (windows/doors). Take in consideration that this settlement usually happens on the first 20-30 years from the date the house was built, depending on the type of soil... some exceptions applies (flooding or oversaturated soil, earthquakes, to name few).
Cost will largely varies depending on what actually the issue is and how severe is. Also take in consideration that the house is almost 90 years old. Usually concrete start loosing its integrity or strength around 50-60 years, but this shouldn't be an issue because this crack is not related to that. You see a lot of small cracks, crumbling of concrete, exposed reinforcement (if any due to the age of the house), and a lot more cracks due to integrity issues.
My recommendation is that you hire a Professional Structural Engineer to take a look. If the structural engineer have some concerns about the crack, he may recommend you to hire a geotechnical engineer or a specialized contractor in foundation/slab repair.
Hope this help and good luck.