I saw that same US Airways ad in a mailing I received for frequent flyers. It's an incredibly good price and Renaissance commonly slashes prices to the bone to fill up slow-selling dates. However, I first received that offer 2-3 weeks ago so I wouldn't be surprised if those discounts are already sold out.
Renaissance provides a great, casually elegant cruise experience and has been cruising the Greece itinerary for a number of years, so it's likely that their land tours are high quality.
Of course, once you add on business class airfare from Florida the trip will no longer be a bargain (unless you're using FF miles), but in my opinion would still be a great value.
Let us know what you decide.