I actually have no idea why Flash is so expensive, and I use it on a regular basis for work.
The funny thing is, Flash is far from the best animation software available, and it's also not the most expensive. Don't get me wrong, Flash is a great tool. It's great for what it's meant to do... animate, but it's pretty limiting in terms of design. As well as Flash, I also own Toonboom, which I believe is what Disney used for some of their films, and it's far more sophisticated. It's also a bit more complicated, I actually haven't had the chance to sit down and work out how to use it yet.
However, there are free options available, if you're interested in something more basic. Have you tried using Pencil?