Plugin buyer here.
I buy around 1 to 2 custom plugin every month, through freelancer. averaging from $30 to $200.
I believe it depends the country you're from, unfortunately (for Tier 1 countries developers) some developers take as low as $10/hour which kill americans, uk, australian or anyone else that want to get into that business.
Now if I were you and really wanted to make money out of it, I would stick to this forum, find methods that are shared and loved by the board, and develop plugins to automate or facilitate that said method.
Good luck.