Well, its not too complicated, but the answer depends on the amount of points you are wanting to convert and the type of award you are ordering.

Marriott's convertion award tables can be found on their Air Mileage webpage. So the answer is you could be paying as much as a 5 points to 1 mile ratio or as good as a 1 to 2.5 ratio.

Of course if you decide to order one of the Marriott Travel Package awards the conversion will actually be 1-to-1 then. And that is why a lot of people think that the Travel Packages are Marriott's best awards.