Limiting how many jobs people can apply to on your website isn't going to stop people from applying to lots of places, it just means that they're not going to use your website that much. And people looking for a job aren't going to drop a lot of money to apply for jobs because they don't have a source of income, which also makes them want to avoid your website. Sure, you could overcome these things if you had an extremely high placement rate, but that's going to be very hard, especially in the beginning.

Your core idea might be a good one, but companies have better ways of making applying more expensive. They have puzzles and automated assessments. Some places basically only hire people who can network in, others require a specific certification, and many bias towards people who have some public work.