What is the typical cost of Summer Course education in the US?
The cost of summer courses in the United States is likely to be as staggeringly variable as the the cost of courses during the semester.  To the best of my knowledge, most universities generally do not change their tuition fees significantly during the summer.

As such, it will range across at least two orders of magnitude depending on the school you are dealing with, and can easily change by nearly an order of magnitude depending on the type of student that you are.  Consider, for example, this table of tuition rates published by the University of Iowa: you will find that summer course charges are almost identical to semester charges, and differ wildly for Iowa residents and students coming from out of state.