I know exactly what you mean - I have a satellite dish on my RV. My service is through Hughesnet.
They have something called the Fair Access Policy, and it truly will close you down to a dialup rate if you download more than about 300 megabytes of data in any given 24 hour period (this is the limit with the plan I have). In the lingo of the trade, this is called "being fapped".
It's a well known policy and nothing a user can fight legally, unfortunately. We agreed to it when we signed up.
Note that I pay about double what you do per month for the privilege of being mobile, and fully half the time I deploy my dish I have some kind of problem getting connected. When I do connect the speed is variable. I'm a very technical and self-sufficient type, yet I've had to call people to help me with the connection (when I was in cell range) a number of times.
I'm sorry to confirm that satellite internet access is half a step from a black art complete with smoke and mirrors, and your access to the internet is at best limited. Yes, that's the state of the art in 2008.
Something that might help you is a tool called FAPMON that you can install and which will monitor whether you've used up your quota. It's available in a lot of places on the net; here's one I turned up with a search:
FapMon !
There's a decent forum for us mobile users, and they do have a good section on the issues specific to the various network providers too.
Datastorm Users Forum
Also, for what it's worth, I believe you can upgrade your plan and pay more per month for a higher quota.
-Noel