The reason something like this would never work as a monthly subscription is that most people don't need to travel (long distances) often. For example, you may go on a short holiday twice a year and have lots of plane, train, boat and tourist stuff to book but then what about the other 10 months? That's $50 for a dormant account. Obviously this could be different for you as you may travel regularly, but for the everyman it'd be pretty worthless.
But there is definitely a lot of space to exploit in that market.
The reason something like this would never work as a monthly subscription is that most people don't need to travel (long distances) often. For example, you may go on a short holiday twice a year and have lots of plane, train, boat and tourist stuff to book but then what about the other 10 months? That's $50 for a dormant account. Obviously this could be different for you as you may travel regularly, but for the everyman it'd be pretty worthless.
But there is definitely a lot of space to exploit in that market.