Shameless plug: also take a look at Veritasprep. All their instructors have personally scored in the 99th percentile on the GMAT and undergo a rigorous evaluation on their teaching skills. They will also let you retake the course until you get the score you are happy with.

If you have the time, how about investing less than $100 on some books and see how things go. If you need to, you can do a course and a rewrite.

I only used the Kaplan guide and scored a 680.

If you are self-motivated, you could buy a few of the Manhattan GMAT prep books and go through them yourself. You don't even need to do the online course because they have published the syllabus that they follow in their classes on their website.

I scored a 660 without MGMAT. For my retake I bought all thier quant books and the the SC book (coz I am really good at CR and RC). This is the cheapest way to do it and if there is sth you don't understand people on the forum can really help you. So if you are self-motivated, you could get all the prep you need for about $250.

Also, one advantage to self study is that you are forced to really try figure out the problems yourself insted of running to an instructor. You learn more this way, I think, and you certainly get more confidence in your abilities when you crack tough problems that you got wrong without any help.