The cheapest pocket I have tried with great success in Sunbaby. They come directly from China unless you buy them used. Buying used diapers can also save you a lot of money.
The ultimate cheapest route is simply prefolds and reusable covers.
AIOs are generally the most expensive and hardest to clean and longest to dry. You also can't increase the absorbent material in most so your child might out pee them as they get older.
I ultimately have a mix of prefolds, fitteds, pockets and AIOs (only newborn size). My husband prefers the pockets and I prefer the fitteds. Since I prestuff the pockets they act like AIOs when in use. I can also easily change what I stuff in the pocket to change the absorbent material based on what my child needs. Fitteds generally are more absorbent so I prefer them because of that.