You can try e-GMAT as a cheaper option. However the best bet is to read the debrief of people and see how they improved. My personal opinion is that Test Prep is not as useful for GMAT Prep as it is claimed - 50-100 point guarantee. Improving verbal is time taking and needs a structured and dogged approach.

1.) For SC you need to make Notes from Manhattan SC for the points that don't come naturally to you.
You need to read the Manhattan SC book to the point you can summarise any chapter to someone if he wakes you at 2 in night. This will require several revisions of the book - i personally read the book 5-6 times to really get a hang of the topics. Don't skip anything - especially IDIOMS.
Also solving OG12 after reading 1 chapter a time is not a good idea IMHO. The reason is OG12 questions test more than 1 concept at a time and you may not be able to understand OG12 explanations due to this. Also your accuracy rate and hence your confidence will suffer. In SC you need to solve around 400-500 problems to get your timing under 1 min.

2.) CR - Powerscore CR is the bible. You need to read this book thrice to retain all the info and apply to Qs. However you can read 1 chapter and solve the selected questions unlike MGMAT SC. You can find the categorized question on the link that is mentioned in the Powerscore CR book.

3.) RC - There is only 1 way to improve RC - Practise, Practise and Practise. You can download 1000RC and solve all the LSAT Questions from there.