There is not simple answer as to how much one should spend on golf lessons and every person and or scenario is different. Many of my students over the years, I am able to solve their issues or get them back on track in a few swings. Many of that group wanted a series of five or ten lessons but the problems could have been solved in ten minutes. Depending on the student, their mind set, skill level and desire each student could come up with a different answer on costs.
Instead of how much to spend, I think it is much more important to first find the right teacher for you and that is not always easy but a very overlooked and important aspect. Once you find the right teacher, then have one lesson and the student and the teacher should agree on a plan for improvement together. At this point the student needs to decide if they are willing to make the changes suggested or not. Too often golfers do not like to get out of their ‘comfort zone’ and make changes that could help them improve.
I like to give all of my students the knowledge to self correct and many can do that well, others want constant eyes on them and reinforcement, so again it depends on the student.
So my answer would be whatever it takes given what the student is willing to put into the plan. For some it could be very little money and for others it could be an ongoing expense.