Neal has stated it well, but I have to ask; when you're asking about how much they make, are you referring to how much they make per reservation or are you asking about how much they make per year as a job?
If you're asking about it based on your interest in working as a travel agent, it all depends on how much effort and time you want to commit to the job. We make a commission on each sale, so obviously our income is based on how much we sell; the more we sell, the more we make. Some agents work part-time and some work full-time. Some work in a store, some work at home, some work for themselves, some work for others, and others belong to a consortium, while others do not. Some sell to the average income clients, while others only sell to luxury-minded clients. (The bigger the sale, the higher the commission.)
I know agents who only make a couple of hundred dollars a year and I know agents who make over $100,000 a year.
In other words, it's not a simple question to answer.