It is the best time to return to Nepal. I think there is a politically stable government now. From what I have seen and heard people who teach biology in universities and tuition centers earn Rs 100,000 per month on average. Since, you have a US degree you will easy get a job and surpass all the competition. If you have a PHD or masters its even better as you can join a good faculty in a good university.
I have one friend who did bachelors in nepal in Computer engineering. He came to usa for masters and returned back home. He got a job in a university within 1 month of him being there. He teaches part time at 4 different colleges and takes private tuitions. He earns Rs 200000 per month. He stays in his parent's home. He pays no rent. You can do the math on his savings.
Another friend who returned after studying physics teaches people for competitive exams and  in different +2 colleges as well as universities. He charges Rs 1000 an hour for personal tuition. He earns around Rs 150,000 per month.
Nepal is up and coming and needs good teachers to meet the demand of students. I suggest you look into teaching if you want to return home.