There are many ways to ask that question tactfully and politely. "Can I ask about your rate?" "What do you usually charge for (this service)?" "Let's talk about your standard rate for (the service)." "What do you usually ask for (this service)?" These are just a few that I can come up with at the moment. Of course there are usually more informal ways to ask that are light hearted and casual like "What will this set me back?" But that one implies that you are expecting the fee/ charge to be high. Hope this helps.