You don't need to pay a book agent. In most cases, agents work on commission, which means they only get paid if and when your book is published. So, if you're working with a reputable agent, they should be motivated to help you get your book published as quickly as possible. Of course, there are always exceptions to the rule. If you're working with a very high-profile agent who has a lot of clout in the industry, they may charge an upfront fee. But in most cases, you shouldn't need to pay anything to an agent upfront.