Memory cards such as Compact Flash (CF), Secure Digital (SD), USB flash drives, and many other form factors have become so ubiquitous, they are available just about everywhere, and have gotten to be amazingly inexpensive. It is not hard at all to find 2 GB cards for less than $20. However… as you start getting into the higher capacity cards (4 GB to 32 GB), you may find that some of the the more expensive cards are more than twice the price of the inexpensive cards of the same capacity. The question I frequently get from people is “Why are some memory cards a so much more expensive than others?”.
To give you a specific example, looking within one manufacturer’s product line (SanDisk), the price difference (MSRP) between the SanDisk 4 GB Ultra III Secure Digital Memory Card, and the SanDisk 4 GB SD Memory Card is about $100 USD. Both cards have 4 GB memory capacity. Both cards are from the same manufacturer. Both cards are SDHC cards. What added value could the Utra III card possibly offer over the much less expensive card?
One of the most important differences is the speed in which data can be written to, and read from the memory card. This difference becomes important in cases such as if you are using a high resolution digital camera (8 megapixels or greater), it would be advantageous if not essential to consider a higher performance memory card. As the resolution of the images increase, the amount of data that needs to be written to the card also increases. In the best case, it takes longer to store the image data on the card, so you have to wait longer between pictures. In worse cases, if the card is not responsive enough for the camera, you will likely encounter instances where the camera reports problems with saving the image to memory. 
Some of the modern digital SLR (Single Lens Reflex) cameras have been designed to be more responsive. In other words, when you press the shutter release, there isn’t a delay before the actual picture is taken. They also require little if any wait time between pictures because the images are saved almost instantly. Further, they frequently offer some form of burst mode that allows you to continue to hold down the shutter release, and several images are captured in quick succession. This functionality requires faster memory cards. If on the other hand you have a lower resolution camera (less than 8 megapixels), and it is one of the lower priced “point and shoot” cameras, you likely won’t get much value from the higher performing memory cards because your camera will be the slower component in the solution.
Another factor that can make faster cards more essential is in the image file format that you choose. Most modern digital cameras allow you to capture images in JPEG at three or four different “quality” levels. The difference between the quality levels is in the amount of compression the image data is subjected to. Compression is used to make the image file size smaller. The amount of compression, also known as “compression ratio” determines ultimate file size. The greater the compression ratio, the smaller the file, and lower compression ratios create larger files. If you choose lower compression ratios, larger amounts of data must be written to the memory cards. This larger amount of data takes longer to transfer. There are some formats that can have no compression at all such as TIFF and RAW. These files can be VERY large. Faster cards become increasingly important as file size increases. With most image compression techniques, image quality is reduced as the compression ratio increases. Therefore, there are reasons for wanting to minimize the rate of compression.
USB flash drives and many of these memory cards can be very handy for transferring files from one computer to another. In other words, they can be a very nice replacement for the old style 3.5″ floppy disks that are becoming obsolete. It takes more time to copy large amounts of data to slower (and typically less expensive) memory cards than it does with faster (typically more expensive) cards. To give some specific perspective to this statement, I did some time comparisons with two different USB flash drives. I copied a folder containing approximately 950 MB of data to each USB flashdrive. In both cases I used the same computer. The first USB fashdrive was a Kingston 1GB DataTraveler which is very inexpensive, and a SanDisk 1GB Cruzer Micro USB Flashdrive which is typically a little more expensive. It took approximately an hour to copy the files to the Kingston DataTraveler, but only took approximatly 20 minutes to copy those files to the SanDisk Cruzer Micro. Both test flashdrives have functioned as designed for me, but one is clearly faster than the other. Both are very affordable, but the Kingston product is less expensive. If speed is of no concern, but price point is, the Kingston product would be the logical choice.
Generally I would say that price point is an unreliable measure of quality or performance. The problem is that the manufacturers of the majority of these types of memory products do not publish speed data. The exception is that with the highest-end product, that has been designed specifically for performance, the manufacturers sometimes do publish speed information. For example, some manufacturers have a special section of their website that promotes their performance line of memory products. The SanDisk calls this their “High Performance Photo & Video” product class.