How big of a deal is this?

It's a very big deal.

SpaceX's entire philosophy revolves around the concept of reusability. Company CEO Elon Musk has been dreaming about reusable rockets since the company's inception 15 years ago, when there was only a Falcon LV—which later became the Falcon 1 Musk proudly displayed on the Washington, D.C. National Mall in 2003. The company's website says reusability is "the key to making human life multi-planetary" because it could lower costs to the point where sending a million colonists to Mars is a feasible plan.

SpaceX first started playing around with reusable rockets at its McGregor, Texas testing facility, using a prototype called Grasshopper. After a series of landing attempts that ended in spectacular fireballs, the company successfully stuck a booster recovery at Cape Canaveral in December 2015. The feat has since been repeated on land and at sea, but SpaceX has yet to refly one of its returned rockets.

Until now.

Where does the flight fit in terms of firsts?

Describing the achievement in terms of firsts requires a few careful qualifiers. Here goes:

This is the first time a liquid-fueled rocket booster used for an orbital flight will be reused for a second orbital flight.

We have to specify "orbital" since Blue Origin has performed this feat during suborbital flights. We also have to say "liquid-fueled" because the space shuttle solid rocket boosters were reusable. Alternatively, we could scrap "liquid-fueled" and add something about the booster landing upright, rather than splashing into the ocean under parachutes like shuttle boosters.

That makes this sound like it's not a very big deal.

No, it still is! Perhaps a better way to describe the flight is:

SpaceX is about to take the concept of reusability farther than anyone ever has.

NASA was in the business of reusable rockets during the shuttle era, from 1981 through 2011. In addition to the solid rocket boosters, the shuttle itself was reusable, including its three main engines. But the program's original reusability ambitions were never realized. Vehicle refurbishment and turnaround times were far longer than predicted, and by the end of the program, the cost of a single launch hovered around $500 million.

Blue Origin has already flown and reflown its New Shepard rocket system multiple times, but those were suborbital test flights. SpaceX is doing this on an orbital flight, with a paying customer.

How much cost savings are we talking?

We don't know, since SpaceX and this particular customer, SES, are private companies.

The list price for a Falcon 9 flight to geostationary orbit is $62 million. Actual launch costs can be higher, depending on how much ground support you require; the NASA-NOAA Jason-3 flight to low-Earth orbit, for instance, cost $82 million.

SpaceX has hinted customers that don't need to throw their rockets away might eventually see 30 percent discounts. A light enough payload allows the company to add landing legs and grid fins, as well as hold back some fuel for the descent.

If you're willing to ride on a "flight-proven" rocket, as SpaceX calls it, President and COO Gwynne Shotwell said you might get another 10 percent off.