As davidash states, Marriott has chosen to use "upgrade" certificates with a set and constant value (i.e. 5000 points.) Depending on the type of upgrade and how long you want the upgrade there is a cost in terms of # of upgrade certs required.

It is just like making a reservation for a room. Do you want a standard room? It has a set price (i.e. $100/night.) But if you want a Concierge Level room, it might have a higher price (i.e. $120/night.) So if you want your standard room for 5 nights, then the price wil be $500 for the stay (plus taxes/fees of course.)

Using the "upgrade" certs is the same. If you want a single level upgrade, it costs 5000pts/night (i.e. pool view room). If you want a double upgrade then that would cost 10000pts/night (i.e. partial oceanview). If you want a triple upgrade that woudl cost 15000/night (i.e. oceanfront). So if you have a 5 night award, a single level upgrade would cost you 25Kpts.