Pretty big difference in far as effectiveness on a 50 vs 55 grind. At 55 you basically have double the health and damage a level 50 would because of the itemization jump on makeb. F2P is also pretty limited unless you buy all the cartel unlocks. I believe you have a 200k or 300k credit cap as well so it will be challenging. At best you can make about 20k per hour at level 50 grinding mobs. Doing dailies probably 100-200k.
At level 50, I would estimate an average grind earning closer to 50k per hour, but other than that, I think salty is spot on. The expansion is free for subscribers now, so I'd recommend going for it.