You can use GWT by itself and support it on your own server. I'm not too familiar with google app engine, but from the website, it says:
"Risk free trial period Not only is
creating an App Engine application
easy, it's free! You can create an
account and publish an application
that people can use right away at no
charge, and with no obligation. An
efficient application on a free
account can use up to 1GB of storage
and up to 5 million page views a
month. When you are ready for more,
you can enable billing, set a maximum
daily budget, and allocate your budget
for each resource according to your
needs."
It sounds like it is free for as along as you stay below the above quotas.