Well, that is the question. Shopkeep says it will give a short-lived doubling of your power, but +30 power isn't double. I'm assuming that the +30 is a statistical representation of a narrative effect. It may or may not be permanent. That said, taking the energy drink only locks you out of one purchase (for Dark479: Shopkeep lets you buy one thing each time you visit him, which you can do a finite number of times, six as of latest update, and between him updating his inventory later on sells seven things total). And you drink it towards the end of an arc, so we'll probably get another opportunity to buy from him between when any power boost wears off and when we need to do more fighting. So it's probably safe to prioritize it.
EDIT:
Ten million. And yeah, even when you have enough money for the stone mask, Shopkeep just goes "oh, I wasn't selling it, I just wanted to see if you wanted it".