They don't have any monthly subscriptions. It's all annual.
If you have a dedicated gps, you can get a chip for about $100/state. It needs to be updated periodically but once every 5 years is probably enough. They don't have satellite, though, just topo. I have a Garmin Montana with a chip that's probably 10 years old. You don't need cell service that way. I like the phone better, though, since it's lighter and more versatile.
I've had a onX Montana chip in my Garmin GPS for about five years now. Montana is a little different than some other states, since their Block Management program pays landowners to allow public hunting--but the properties which sign up for Block Management vary from year to year.
As a result I update it every year when the new BM info is available.
When I first got the GPS and onX chip I could take it back to the local store where I bought it and have them update the chip, but three years ago onX changed its policy, and I had to go on their website to update. It wasn't hard, but the next year they automatically updated it and notified me via e-mail, charging the same $29.99. Same deal this year, which is pretty handy in situations like Montana's.
