I'm a huge fan of sand in the run. In my coops, as well. I'm very sensitive to smells. There's nothing smellier than accumulated chicken poop. Sand allows me to scoop the poop, eliminating all together the smell of chicken poop.
You want to know what else scooping poop eliminates? Flies. Zero fly problem when you scoop the poop every day. Healthier chickens, too. They aren't wading around in accumulated poop which gives their immune systems a big break.
I buy construction sand at the local gravel yard for about $15 a half yard which I haul home in my light pickup. A yard of sand will do a 400 sq.ft. run.