A:That actually makes sense. The Grand Canyon is in essentially a high desert. But August is the monsoon season. You can expect some thunderous lightning storms and accompanying rain.
What kind of rain suit is an open question. You're really not going to be wearing it to hike around very much, so in my view you do not need an expensive set of breathable jacket and pants. Really, Helly Hansen's Essential Jacket—made of fully waterproof-coated nylon—is perfectly adequate at $65. The matching pants are $36. These are among the most totally waterproof garments on the planet. No, they do not breathe. But if you're just sitting in an open raft or around camp, trying to stay dry, that hardly matters. I wear Helly Hansen stuff all the time.
The next step up would be one of today's generation of reasonably priced waterproof-breathables. The $99 Marmot PreCip Jacket really defines this category, and is a decent piece. The pants are $75. Outdoor Research's Revel Jacket ($165) adds stretchy Pertex fabric and adequate breathability in a very nicely designed piece. Fully taped seams, handy pockets, folding hood, and more. The pants are $135.
If you feel like spending real money, you can do that, too. REI's Shuksan Jacket is a superb option that uses eVent fabric, which is super-breathable and waterproof for $300. For the same price, the Arc'Teryx Alpha SL uses Gore-Tex Paclite for a light, breathable piece, albeit less durable than hearty shells. With either, Arc'Teryx's Alpha SL pants ($199) would be fine.
Hope you have a good trip! I'm heading to the canyon in April, but to hike, not raft. I'm getting pretty fired up about it. It's an amazing place.