How much, on average, does a professional submissive make in a year?

This is almost an oxymoron. If you are a submissive then you wouldn't be demanding money (that's a dominant trait) so you would end up with nothing.

Actually, I've never heard of professional submissives. Submissives tend to have the greater need, to be protected and nurtured and disciplined, where dominant figures would tend to look for submissives they don't need to pay, except to buy presents for being well behaved.

There are professional subs. It's more common to be a prof. Dom, but yes there are prof subs, for example http://www.professionalsubmissives.com/ There's a list of some there and their rates. And here's an article on one in particular http://www.villagevoice.com/people/0607,bussel,72157,24.html

Some Dom/mes send their submissives out to gain experience and make money for the Dom/me.

Its something the sub enjoys as far as the experience and providing for his/her Master/Mistress.

I am a professional submissive in Melbourne Australia, specialising in elegant submission combining pleasure and pain, belts and sensory play.

I was a lifestyle submissive for 2 years, and love submission so much I chose to pursue a career in it (despite already having a perfectly well-paid job in management).

I think that dominant men who pay for a professional service enjoy the discretion in a fully set-up environment with all the equipment they need (which is normally expensive to purchase and takes up space). I think they also appreciate intelligence, beauty and seriousness of a professional submissive to play with.

The going rate in Australia is around $420 per hour in Australian dollars.

It is a demanding job, both physically and emotionally, and I would advise anyone looking at pursuing professional submission to ensure they are in good form. (It is not a job you want to be doing if you're not feeling whole in of yourself.)

It attracts misunderstanding by many, and a strength of character within the submissive.

Submissive and trainee switch Louella.

Think long and hard before you sell yourself sexually. There has to be better ways of supporting yourself.

That would depend on how in demand you are.

With this profession, it is what you make it. There isn't an average "per year" because it depends on a lot of different variables. Yes, it depends on how much you're in demand, but also how heavy your sessions are, how much you're willing to travel, how much you want to work, etc. As a NYC-based sub working out of a set Dungeon (no outcalls at the moment), my rates are between $200 - $350 for incredibly light to very heavy sessions. Outcalls are more, and may require an hourly minimum.
Also, to the people who think this is "selling yourself sexually" or that pro subs don't exist - these people have no idea what they're talking about.