I think amateurs need to first understand that there isn’t a magic number of lesson one needs to take. They should first create a goal and a timeline in which they wish to complete this goal. Then create a budget on how much they would like to spend on improving their games. Lastly, they need to do some research on instructors in the area that both fit in their budget and can help them reach their goals.
So in short, I don’t think there is an “exact amount” someone needs to spend to improve; however, to get the best bang for their buck, they need to find someone qualified and who meshes well with their personality. Before I work with a new student, I always conduct a quick phone call to make sure we are a good match and that I can help them achieve their goals. If I feel that we aren’t a good fit, I usually recommend one of my other local colleagues who I think would be a better match.