For some parts, their prices are very predictible. Take Intel CPUs for example. Intel drops their CPU prices one rung each quarter (~ every 3 months). A chip that costs $417 now will cost $278 after the price cut. A chip that costs $278 now will cost $218 after the price cut. A chip that costs $218 now will cost $178 after the price cut, etc. Note: this is the cost to companies that buy from Intel. You don't buy from Intel. Thus companies will pass the savings on when they choose to pass them on. Typically the result is that prices gradually drop just before Intel cuts prices, they plunge, and then they slowly level off after 3 weeks or so. Intel's next price cut is late Jan / early Feb.
Other components are completely unpredictible. Memory is a great example. Memory costs are highly volitile just like the stock market. One day memory prices could be up, and the next day they could be down. Supply and demand plays a big role. If demand is weak for the holiday season, prices will drop. If demand is strong, prices may rise. Like the stock market generally goes up in the long run, memory prices generally go down in the long run. But on any specific week or month, no one really knows (unless there was some major news like an earthquake distroying a memory factory or some major lawsuit).
Hard drives are in between. They are more of a steady price drop, but with some fluxuations.