The method you choose to prepare for bank exams should depend on your capacity to study, allot time & be dedicated in your preparations.
If you are new to competition exams & havent attended even college placement exams, you can join a good coaching centre in your city.They would help you undertstand the pattern, provide study materials & will have staff to explain concepts, clear doubts.Since the classes would be conducted in fixed times, you will be present there everyday to practice more questions.
But most bank exam coaching centres/ institutes charge somewhere between Rs.5000 - 15,000 (based on city, demand & brand name)
Self study option is cheap & economical.Bank exam books such as guides, previous year solved question papers,practice workbooks etc., are available online nowadays which start from Rs.150 onwards.
You wont spend more than Rs.1000 even after buying 3 or 4 books.This will help you in practicing more questions as done in coaching centres.
The only requirement when you choose to prepare on your own is Time.No matter what, make sure to spend atleast 2-3 hours everyday for studying.Practice will make you confident to answer any type of question in the written exam.