I can only speak about Dish. They have a "pause" option so that you don't pay when you aren't using it. However, there is a $5 a month charge while on pause. (beats a contract in my opinion). I have been happy with their service and customer service.
I have two antennas. (can't speak to roof antennas, other than they can't be moved to account for trees and things; someone else will have to chime in)
One is the Tailgater, for short stays, which I simply set out and plug in, and it finds the satellites by itself. I usually set it on one of the slides since it only weighs about 5 pounds. It will only run one TV in hi-def, but you can run a second tv in low def.
The other antenna is a tripod antenna, that I have to set up and aim each time. It takes some getting used to and can take time, but worth it to me in the long run. I use it for long term stays, and I can run up to 3 receivers off it, all in hi-def. I have two separate wire runs from the antenna to two separate receivers. I don't use the RV wiring since I always seems to have issues when doing so (cheap cable, loose connections, etc)
Cost for viewing depends on how many channels you want. Go to their website and check out the packages. I pay about a $100 a month for what I watch. About the cheapest package you can get is going to cost about $75 after taxes etc, and you can easily top $200 a month with lots of channels, movie channels, sports packages, etc. Up to you.
Cost for equipment depends on what you use.
The tailgater is about $375.
Tripod antenna about $150.
Another $50 for the satellite finder kit (for tripod antenna)
Vip211 receiver is about $75. Wally receivers/Hoppers are $100 plus, I think.
Hope this helps.