If you are tight on money, I suggest you study on your own and spend at least 8 hours per day on GMAT for 4 to 6 weeks. I have never took any preparation courses. English is my second language also. To prepare for the exame, I did most of the materials available on the market and some extras not on the US market. (to see a list of the materials I used, you have to check the buy/sell section of this forum, it's way too long for me to post it here) After one and half month of study, I scored 760 on first shot. I heard from friends that if you are already very good at quan, then you don't have to join any preparation course. So I didn't. As a result, I got 50 for math and 42 for verbal on the exam without any course and saved tons of money. I also heard that prepartion courses deflate your score for the diagnositc exam in order to make you very satisfied with their results.
Good luck on your exam. Oh yeah, make sure you drink red bull during your math to verbal test break. I find it helpful.