Think to yourself about these questions to help you better price your services.
1) What is your competition charging? Like @paul_g said, don't undervalue your services just to undercut your competition. In some cases to the consumer lower price = lower quality
2) How much does it cost you perform the said service offering? This is an area I think a lot od detailers starting out don't think about. Think about your fixed and variable costs (supplies, gas, your time, website, etc.). How much do you really need to charge to be profitable.
3) Think about what value you're bringing to your customer. Are you restoring their car so the paint lasts x amount longer? Are you increasing the value of their vehicle? If so, by how much? What are their alternatives besides using you or a detailer in general? Do nothing, get a new paint job?
If you really hash these out and think about your cusotmers, you'll be able to almost read their minds and truly bring them value. After all, that's what their paying you for...you're bringing them something of value.