As a professional pet sitter, I have joined the professional organization, Pet Sitters International out of King, North Carolina. From information they've obtained from their members, the average pet sitter charges approximately $17/visit. This information is collected from members around the country, and the cost of visits will vary due to region, economy, services provided, years of experience, etc.
One of the first replies I receive as a pet sitter is "how fun." Not quite, at times. I've been in business 22 years and have seen the industry change dramatically. As in any business, there are slow periods and periods of the year that are overwhelming with business. Many come into this business believing client's will knock down their door immediately, and it's just not true.