If he really wanted to save the Komaba farm financially, he should have asked Tamako for advice, not the teachers, his idiot roommate, or random club members. She's absolutely ruthless in business. Whatever solution she came up with would have been hard for the Komabas (and Mikages), but it probably would have worked. If asked, she'd either help for the challenge, or let Hachiken know how hopeless it was in a concrete logical manner so that he wouldn't have to have been so torn up about it.
Hachiken may be the smartest all around person in his class (or maybe even the school), but his classmates destroy him on an individual subject basis, whichever their specialty happens to be.