Should it cost less than $12,000 to replace a bathroom and cast iron pipes in 1960s home?
Impossible to know what is involved from the vague description. I can't even understand whether the bathroom is above the basement or in it from your description.

Generally speaking, minor structural repair on a house might cost anywhere from $500 to $2,500. Replacing free-standing sewage pipes might cost $400 to $800. If things (like the house itself) need to be lifted or moved out of the way, costs can go up. Usually minor structural changes can be made without lifting though.