This all depends. Is this for a trip to the air port or a night out on town?
They usually quote flat rates to airports plus tip and possibly a gas surcharge. If it is hourly you will be quoted a rate per hour plus tip, a fuel fee and there will likely be an hourly minimum.
I'd figure out in advance what you need. Then call about 5 limo companies in your area and see what they say. Be sure and ask them what years their cars are, how well they are kept up and how long they have been in business.