The cheapest way to prepare for these tests is to take the practice tests online with the college board. Sign up for their daily questions and join groups online that are preparing for the tests. The next least expensive way to prepare is to purchase a College Board preparation book from any book store and study it.
There are many tutors that will help as well, but you will have to pay per hour. There are also many programs, one I am using is from collegeprepgenius.com, but there is also a cost.
The main point to the tests is to see if you can think critically and answer the questions quickly. Read the answers first and use common sense.
Blessings on your preparations and tests!