I am wanting to find the best RAM for its price, but I'm not sure if I should focus on getting RAM with low CAS latency or higher operating frequency. Please tell me which you think is more important and why.
The real question is what build are you considering? Neither AMD nor Intel need RAM faster than DDR3 1600 in most situations. For gaming faster RAM will give you exactly 0 more performance. In things like massive multitasking and video rendering there might be a very small advantage in the neighborhood of 1-2%. This is due to the CPUs having the memory controller built in. They just do not need anything faster to give maximum performance. The exception is when building using an AMD APU. The onboard graphics of an APU will use the fastest memory you can give it.
As to your question DDR3 1600 cas 9 is roughly the same performance as DDR3 1866 cas 10. So both cas latency and frequency matter. With DDR3 as cheap as it is right now and 1866 kits almost the same price as 1600 kits a DDR3 1866 cas 9 kit is about the best "bang for your buck" right now.
If your cpu is amd, then it will benefit from faster ram.
Sandy and ivy bridge cpu's have excellent ram controllers that keep the cpu fed with data from any speed ram. . If your cpu is sandy or ivy bridge, then ram faster than 1600 is only marginally helpful.
Here is one report from a year or 2 ago: http://www.anandtech.com/show/4503/sandy-bridge-memory-scaling-choosing-the-best-ddr3
Ram performance is somewhat related to the cas divided by the speed, resulting in almost equal performance from many different combinations.
My take is that more ram is a better value than higher performance ram.