These are good questions!
As far as I am aware, a good source for cost of structures (among other things) is RSMeans. I am sure there are others as well but this is generally the one I hear mention of. Unfortunately it’s not free, but it seems to be the gold standard as a cost estimation reference.
In terms of knowing which structural system to use in a given scenario, you’re right in saying there are many pros and cons to each that offer guidance. IBC also provides some additional clarification on which systems can be used in terms of construction type and fire protection. As a simple example wood frame structures can be used for residential projects but are typically not appropriate for commercial projects.
Another component that is often helpful in determining structural systems is span distance. Although not technically listed as a reference for the PPD exam, The Architect’s Studio Companion has a fantastic Practical Span Ranges for Structural Systems chart that breaks down how far certain materials can span.
Flexural Stress is a material property. Sometimes it is helpful to think of flexural strength which is the maximum bending stress a beam can handle. In terms of an example, THIS YouTube video (although VERY dry) is super detailed in explaining the stresses that can occur on beams and may be helpful.
Hope this helps!