I was lucky to get a Mobley when they had a good deal, thanks to folks here on the forum posting about it.
Before that I used my phone's built in hotspot. With a little bit of rationing on TV watching, I was able to keep it down to 10 gigs a month. When I'm camping I don't really watch that much TV.
When I want to download a large file, like something on video, on campground wifi, I do it in middle of the night. I tend to stay up later than other campers, so I might set it to download at midnight and then turn it off when I get up at 3 or 4 in the morning to pee. Or on an early bird schedule, you could do the other way around -- turn it on in the middle of the night and turn it off when you get up at dawn.
You could also do like RV Sue, and buy a VHS player at a thrift store and buy super cheap VHS movies as you travel around :).