Very low. I was lucky I got one on my first try but my friend Oran Strongrunner was farming him for almost five hours! But I don't think it will be worth it. Lucas drakebloom level 45 Ice wizard Oni slayer
most people say its hard, I got mine on the first try after going there after boredom lol. It's about the same for all pets. Think of the drops like wheel of fortune. Some panels are big while others are small. Getting a specific one that is small won't be easy, but can be gotten quick, though rarely.
I guess that question is a matter of opinion, because for some people it takes 1 try, and for others, 100 tries! I was lucky enough to get it on my 7th try, but there will always be the person that keeps trying, and trying and trying, until they get it when Celestia comes out and nobody cares anymore. So you can forget about the Crabling and move on, or you can be the person that kept trying and got lucky. It's your choice, you can do the smart thing, or the wasteful thing. The only question is, which is the smart thing and which is the wasteful thing? It all depends on how lucky you get!
I have been farming with my friend for two days and I just got it today. My friend wasn't so lucky... :? I don't think it's really worth it, but you can try if you want. The good thing is it hatches in 5 minutes.