Modular vs. Stick-built
Some modulars use low cost mobile home type trims and cabinets or doors, while others use materials that can rival any site built custom home so you may not be making equal comparisons.
As for the home itself, a number of todays modular homes would not be ourwardly distinguishable as a modular. They are not all just boxy ranches with low pitched roofs. In fact, many of the homes that leave the plant in which I work are sprawling two stories with walk up attics, 9' ceilings, and 12/12 roof lines. Generally, most people confuse modular homes with manufactured homes. They are two compeltely different structures built under completely different sets of systems approvals. Manufactured homes (trailers, basically) are built under HUD code, while modular manufacturer built houses according to systems approval packages based on each state the home is going to.
So, my long winded answer to your question would be that there really is no rule of thumb when it comes to modular resale. The cost savings came during the construction process.
Hope that helps.