We have a tripod system and those are available used for well under $500 if you are patient. What you need to watch out for though is that you need a HN series modem. Some of the older modems, while they may still be in service, cannot be reactivated on a new account. I'm not sure if anyone is still selling new tripod systems, likely they are and they will be in the neighbourhood of $2000. Rooftop systems are $5k to $7k new and they too are available used, often under $2500. Same restriction applies to the modem for either system (its the same modem).
Personally I wouldn't want a rooftop system. There's times it would be really nice - you pull into a Wallymart lot for lunch, clear view of the sky, push the button and go online while you eat lunch. But .......... pull into a well treed campground and good luck finding exactly the right site with that perfect hole in the foliage to let you shoot out to the south. Not so much a problem where we are now (southern Texas) with the satellite shooting up at 50 degrees but try it in the PNW or western Canada when you are down to 30 degrees or lower. In those situations I can wander around my site with my aiming tool looking for a little hole in the trees. Then I can set my tripod exactly in the right spot to shoot out through that hole. I'm actually going to try using my tripod system on the boat when we get back out there in a couple of weeks - got me 300 feet of siamese coax in the bay right now just waiting to go to work.
Don Bradner runs this
web forum and its the bible for all things Hughesnet related. If you go ahead with a Hughes tripod don't hesitate to contact me - not that I know all that much but I've used it for over 4 years now so I've pretty well done everything wrong that its possible to do. All you need to do is do what I didn't do and you'll be good to go.