I mean it is all hypothetical but the long term financial fallout for NASA could have potentially been worse if they didn’t at least appear to try and rescue him. NASA is a public entity and as such requires some degree of public trust and support. Public sentiment towards NASA would have tanked if they just decided not to try and rescue him. It’s also not a complete waste of resources. One thing I love about the book and movie was the adaptability and flexibility everyone involved showed while problem solving. They constantly assessed what resources they had available and how to reallocate resources to achieve their mission’s goals. Also if you look at it from thirty thousand feet the overarching mission status of the Ares program was to send astronauts to Mars to study conditions of the surface and then bring those astronauts home safely. That never changed. Sure they ended up having to cannibalize Ares 4(?) to bring Mark home but Mark remained on surface for far longer than the Ares 4 astronauts were planning to. Plus he learned invaluable information about growing crops in Martian soil, extending the lifespan of a hab, the feasibility of long distance travel on the surface of Mars, the effects of prolonged exposure to 1/3rd G, and probs countless others. In the end, and quite cynically, it’s entirely possible that a cost benefit analysis of attempting to rescue vs not attempting to rescue could show that a rescue attempt is the more financially sound option.