I have a set of really nice Axion Monsoon Oceanweave 45 (for total liters) panniers, those are completely waterproof by themselves.
I do, however, still line the bag with a heavy-duty trash bag, why? because I don't want something leaking from inside the bag ruining the inside of the bag liner, or stinking the bag up, plus it's added insurance against water coming in from the outside.
The last thing I want when touring is finding wet anything, including something leaking inside getting on stuff in the bag, so I have leakables in Ziplock plastic bags, I have stuff that I don't want to get fluid on also in Ziplock bags. A bit paranoid? yes, perhaps, but I don't want a rude awakening either. Even though the Axiom bags are water proof I bought a set of rain fly's for it only because the rain fly is a neon yellow so it stands out well, and it will help prevent abrasion damage to the bags themselves.
But those Axioms are no longer cheap, I got a great price for mine but now they're rather expensive. So a good alternative is the Vaude Aqua Back Plus panner, they sell for around $145 for the pair, they are said to be waterproof with a roll down system, but use trash bags and Ziplock bags for reasons I stated. These cheaper bags are a bit smaller than mine, but they'll do the job if you pack light, I don't fill mine all the way to the top anyways, so a smaller bag should be fine for you.
Also keep in mind when buying panniers is how long do you want them to last, if you plan on using them a lot, or go on a single many months' tour you should consider a better built bag then the cheapest thing you can find, but if you are only going on a weekender than the cheaper ones are just fine. There are certain things when touring you don't want to go cheap on, tires, racks, and panniers are three such things.
