No rain cover will keep your gear dry in a downpour, so I don't bother carrying one anymore. Water is insidious, it will find a way in. A bag liner is much more effective. And it's cheaper and lighter.
I use trash compactor bags because they are thicker than regular trash bags. I use two in case one gets a hole in it, but also because I use them for other purposes at camp.
When using a tent or cowboy camping under the stars, I use two as a ground cloth. When sleeping in a hammock, I'll lay one on the ground beside the hammock to stand on when changing clothes or when getting in and out of my sleeping bag.
I sometimes use the other bag to store all my gear in at night to keep it dry or to waterproof my food bag hanging in a tree.
Pack Rain Cover
A pack rain cover will cost $25 - $40, but if you want your gear to stay dry, you'll still need to line your pack, or use several dry bags which can cost $10 - $30 each. I bought a pack of trash compactor bags for $10 in 2006 and didn't use them all until 2012, and my gear has never gotten soaked. So, if you're looking to save money, this is an easy way to do it without reducing any comfort, convenience, or safety.
When I want more protection for more sensitive items, like a digital camera, or when I want to keep small items organized like my first aid kit, I use Ziploc Freezer Bags with the double seal. Avoid generics, Hefty brand zip locks, or any bag with the zipper top, they can't be trusted. ZipLocs are cheaper and lighter than dry-bags, especially lighter than clear dry-bags, but admittedly, ZipLocs are not as environmentally-friendly, so someday I may switch. That being said, nothing is better for the environment than a population that has developed a deep fondness for the outdoors. If finding ways to save money gets more people to try backpacking, then I believe in the long run we'll all be doing more to protect it.
At least that's what I tell myself to reduce any eco-guilt.
Anyway, enough preachiness, every once in a while I get a small tear in the liner, as you're probably assuming, but it doesn't happen often. When it does, I patch it up with a small piece of duct tape, which I always have wrapped around my trekking poles.
One final note, if using trash compactor bags for a ground cloth, be sure to fold in the parts of the trash bag that stick out under your tent. Otherwise, it will divert rain water under your tent, which defeats some of it's purpose.