I've been thinking about getting a gimble for my drone but I'm not quite sure if I should or not. Is it worth it to have one?

would depend if you want to take pictures/video of what you are flying over and better still to be able to see what you are flying over as you are flying over it!
How much for the gimble + camera vs. how much to buy a new Phantom 3 Standard with a camera already on it?

It makes all the difference in the world to me being able to see as I fly what video or photos I am taking. I fly a little with FPV but mostly use it to plan and do my shooting. I count think about having the quad with out it . Once you have the onscreen your hooked.