in using camera, there is no beginner.
a battery grip for XXXD series should cost $100-2xx (low $200).
i already told u, XXXD series the camera body/grip is the same. what makes u think 600D will becomes bigger in size and grip when 450D-550D is ard the same/similar size? I can tell you straight now, u will go back to square one if u upgrade to 600D cos only the cam features will be enhanced, the body cam size will still be similar to your 550D (99% chance)
u can go rent a battery grip for your 550D to try it out.