Actually, standard audio modem transmission has been achieved over GSM. But due to the very mentioned nature of GSM compression you're stuck with ridiculous ancient speeds like 150-300 baud, and even that with high data loss (recoverable thanks to TCP/IP though)...

Yet another option of data transmission is WAP, I'm not sure what does it overlay on "cellular" side, but on the server side you need just software on a server attached to Internet.

With ~100 bytes SMS is a neat idea, but unfortunately unreliable - all SMS are sent on "best effort" basis, that is an SMS may be delayed by hours or even cancelled in the SMS center.

But I think most of your GPRS worries are unsubstantial. In all cases I know of, GPRS is a service paid per byte, not per connection or per minute. Stay connected a week, send 64k, you pay for 64k. (they take their cut in some overhead though, like minimum packet size/padding, TCP/IP overhead and such, so actually your 100b message will end up as some 10k total of data transferred in the billing, but it's still very cheap)