Admittedly for $100 you might have to choose one or the other (though I used a $12 ergonomic mouse from Amazon for years until I switched to a trackball, and I loved it) - but if your job is computer based, you really should consider switching to ergonomic equipment.
Your average keyboard and mouse setup is absolute murder on the wrists in the long run - if you spend more than a couple hours at the computer every day without ergonomic equipment and your wrists don't hurt, then it's only a matter of time.
Granted, it's not just about buying a cool new keyboard and mouse - you also need to cut out bad habits like wresting your wrist on the table while typing and so on, but a good KBM will help you build those habits naturall