Estimating usage has to do with what kind of files you need to work with and applications you want to use. Don't expect a lower-end Palm to hold endless pages of notes. Units with less than 16 MBs may prove disappointing in just a few weeks. Color displays and size of display can increase price dramatically. Flash memory and Flash operating systems can be expensive.
More RAM costs more.
If you do, you'll find yourself replacing the handheld sooner than you should have. A small computer typically small enough to fit in the palm of your hand. They are used to keep and retrieve information on the go. This means the device can grow as the OS is improved. It is used like a mini-hard drive would be in a desktop computer.