I use the recycled rubber mulch. It looks good, keeps it's color, lasts 10+ years, stays in place, and you only need to put it a few inches deep. I have had it down for 3+ years and it has been great. I got the DuPont version from Lowe's. It is more expensive up front, but about equal over it's lifetime and requires much less work (replacing, keeping it in place). It is definitely worth a look, in my opinion (granted I don't have as large an area as you do, but I did use ~1.5 cu. yards.

One thought is what one of my neighbors did with his yard. Our city does a lot of tree work with the city-owned trees and offers a chipping service for homeowners for their fallen limbs and prunings. They offer the chippings free to pick up, and will even deliver them free (min. 3yds). The chips are a lot bigger than regular mulch, last a lot longer and do give a natural look. If that look appeals to you (my neighbor likes it!) you might check into whether or not your city offers such a program.

Or, you might check with tree service companies in your area. They might be happy to have a place to unload some of their chippings at little or no cost.