I'm not sure what'll happen to the price but generally Apple keeps the US price the same. Though it did go up with the last refresh. It'd be nice if it came back down to $599 USD.
Current models would drop in price to clear. As for refurbs they might not drop in price till there's good availability of the new models.
I doubt they'll lower the price of the new mac mini, but you never know.
The current model should drop $50-100 and be moved to the Clearance section of the Apple store online as soon as the new ones come out. That won't last long, only until they sell off remaining stock (in the past they're sold out online in a month or less).
You might be able to find a refurb of the current version, but those prices and availability harder to predict and are hit and miss.