I went to work in Australia on a visa through BUNAC but already had a job lined up, so didn't have to spend any time job hunting or moving around for random jobs like a lot of others. I'm getting another visa now through them to go to the UK in the spring.
Yup, your understanding is right, basically "pay to play." So if you know you want to work and need a visa, they're the people to go to. I think the prices are similar to any competing companies.