Stringers that are built out of light materials certainly do crack from stress over time. 100 year old stair cases in historical buildings are usually built very well, or they are not open to the public. Old staircases creak and pop as the parts move under the load of use. When you look at a stringer, measure the stringer at the deepest part of the cut. This is the effective dimension of the lumber. The more material you have there, the better. This is the back bone of your stairs. I prefer mortised side stringers. They are MUCH stronger.
A cracked stringer is like a cracked ladder. There's no if it fails, it's when it fails. The same can be said of risers that support the treads. (no center stringer) When the nails driven from the back into the treads fail, the treads get flimsy. It's pretty difficult to repair stairs like this especially when it runs between walls.
I did some repairs on a set a while back, removed a wall beside them and set a load bearing beam across the opening. The rail ran up into the beam and the ballusters continued. It made the living room seem much larger and more interesting. The stairs became a part of the room where it had been enclosed in the hallway. It was a big job compared to what the home owner thought it would be. I was 3 days with help and 3 more days alone including shop time and drywall work. I made the ballusters and the rail.
Cost? In your area, I couldn't even guess.
Parts pricing varies area to area. Treads are $20 to $30 here and ballusters are $5 to $12, depending on paint or stain grade and the style of turning and material. Railing is always expensive. Newell posts are available in many different styles.
If this will add to the value of the home, it's worth considering. If not, forget it. The house I did this in, had a big old picture window in front. You could see the staircase as you drove by. When they listed it for sale, it sold within a month in a small town.
[URL="http://www.houserepairtalk.com/announcement.php?f=39&a=6"][size=3]The Ten Commandments of House Repair Talk[/size][/URL]
[URL="http://home.bellsouth.net/p/s/community.dll?ep=16&groupid=289528&ck="]Square Eye's home page[/URL]