Adrian Dingle, Author of Crash Course AP Chemistry prep book
Honestly I'd be EXTREMELY careful about looking for 'the cheapest' tutoring, since you often get what you pay for!
Also, many agencies employ inexperienced people, who are simply looking to make a quick buck and that have no idea about what it takes to enter into a relationship that is the basis for successful tutoring. Here's a more comprehensive point that I make on web site in the Tutoring section about graduate students (not teachers) acting as tutors.
"Typically, otherwise very intelligent graduate students with good subject knowledge, will lack three crucial qualities. Firstly they are likely to be very inexperienced teachers. Secondly they may have a very tenuous grasp of what is required of high school students and as a result fail to pitch their instruction at the correct level. Thirdly they lack the formal professional qualifications and training that an experienced teacher will possess. The combination of these shortcomings is likely to result in tuition that is relatively ineffective."
In short, I would reach out locally to schools and see if they have a list of approved tutors first, rather than going to an agency where almost anyone could be employed. Many of those local tutors (like me), offer online tutoring for remote based students.