A structural home inspection is a no brainer. The cost of an inspection is small compared to the cost of most legit problems, including your safety. Better to walk away or make the owner pay.
The cost for an inspection will depend on the scope of work and the cost of living in your market. Definitely try to find a licensed engineer with more than 5 years of experience (engineers know nothing coming out of college) and with experience with residential remodels and ideally historic architecture. The reason is that a lot of factors may come into play, including the method of construction and construction materials at the time the home was built. Residential structural is actually more complicated than light commercial in most cases, and this type of inspection requires even deeper knowledge than design.