The convention center is also very close to the Auraria campus - University of Colorado at Denver and other colleges - so if there is a free parking space or two (and I can't imagine there is) I can guarantee you a college student has already found it and is occupying it.

Surface lots are probably your best bet. 14th Street and Glenarm or 14th Street and Tremont have lots that are often $5 a day, but they of course may be higher if there's a saturation event at the convention center.