Hello
Thanks for your question.  By ‘cost’ do you mean how much do you have to pay?  Generally, there shouldn’t be an ‘upfront’ cost.  Rather, agents take a commission from an author’s earnings once the author has a book deal.
If you have a look on the Society of Authors website, it will give you a lot of objective help and advice on what to expect from an agent, what is standard within the industry and what sort of commission rates etc are standard amongst agents.
Polly