I got word that the Oracle Java certification exam is pretty cheap to take for the next four weeks ($25 as opposed to almost ten times that). What's the chance that a junior dev working currently with Java (aka me) can learn enough from the official Java certification course in the evenings until then and pass the exam?

Depends.

Hard to tell.

I don't think certificates are useful at all...

But the problematic part of the Java certificate is that it can touch some not so simple things, eg. completable futures.

It's the 11 version I think?

Then modules / JLink and other stuff will be hard, too.

Not undoable, but without experience hard to grok

I've secured many jobs with various certifications. I don't have Java experience enough to speak towards that cert specifically. In the end, is it what you want to be doing with your career? If so, go for it! If you fail, nobody will ever know you took the test, right?

Do you have a burning desire to be a cog in some corporate consultancy?

Certs can be a good way to jump into a new technology or area, but for the most part they're useless imho. They go out of date quickly and only show you're good at passing an exam - they say nothing for your real world experience. I pretty much ignore them on CVs, and most people I know do the same.