Flagstaff is mountain country. Paria is canyon country--a bit warmer on average, to put it briefly. However you can still get drops down to the 30s at night there, maybe even lower if it's October or later. On the whole, though, your cold weather will almost certainly be restricted to nighttime and the morning. I would bring whatever clothes you're going to hike in, maybe a shortsleeve shirt and pants, maybe a light longsleeve shirt and shorts. Then add in a down jacket. There are a number of ultralight options in the 12-16 ounce range that will keep both your backpack and your wallet nice and lightweight. If you are the kind of person who likes horribly frustrating endeavors like making jigsaw puzzles when two or three sets of puzzles are mixed into the same pile together, you might also try making your own jacket for less $$ with materials from thru-hiker.com. It won't be pretty, but it will (probably) be really light and warm.
The idea here is that most folks don't need much insulation while moving, even in 30 degree weather (a windbreaker helps, though, and in colder weather you add in more base layers). If you're actively moving, though, you can generally stay warm. Now what happens when you stop for camp and the sun goes down? You can't stay warm by doing jumping jacks all evening--or rather you might but I doubt anybody would seriously do this. Therefore you extract your puffy down jacket from your backpack like the pro that you are and warm up. It might still get colder that night, too cold for your ultralight tissue-cloth, half-inch-thick, underfilled "coat" (at some point you WILL curse yourself for putting so much importance on ultralightitude when it comes to down jackets). That's when you hop in your warm down sleeping bag and wait it out until morning when the weather warms up again. Once you're on the trail again the next morning, you'll warm up again.
Just make sure you bring an adequately rated sleeping bag and can keep it dry just in case of a rain.