What country are you hunting? I imagine if you are hunting SE Alaska or the Pacific Coast your pack may gradually soak through but in Colo, Wyo, NM, and elsewhere it "usually" rains 1 day and is sunny the next. I have a decent pack and haven't ever worried about a pack-fly...even in Alaska.
With that said, I was on an AK dall sheep hunt a couple years ago and my buddies pack soaked through at the zippers while mine was dry as a bone. He just bought a brand new cheaper pack while I was using a high quality 10 year old pack....you get what you pay for in packs! One thing to consider before your hunt is putting some junk in your pack and spraying it down with a garden hose, sprinkler, or in your shower. Time will tell if there are any leaks and it's a lot better knowing before than during your trip!
If you want to go the super cheap route a thick, plastic garbage bag is pretty cheap...although the plastic may rip on brush. A third option is what I used in Alaska as a backup. I bought several super light dry bags. I used one for my sleeping bag, 1 for camera gear, and another for clothes and other gear I wanted to make certain stayed dry. I'm pretty impervious to super wet weather with my waterproof pack plus the dry bag senerio.
A pack cover would be quick and easy to use but I've always gotten along without one. I guess if I hunted on the coast a bunch I'd probably invest in one!