When it comes to home improvement projects, it's sometimes about as cost-effective to hire a professional as it would be to do it yourself. True outdoor fireplaces can cost thousands of dollars when you factor in masonry, gas line installation and other construction issues, which contractors can often handle more cheaply and efficiently than a regular Joe. Just make sure to obtain multiple quotes to get the best bang for your buck. Project quotes are generally free, so you'll surely get a lot of great ideas from the experts you consult without spending a cent.
Another major plus to consulting a professional is that they tend to be in the know about local permit requirements and other regulations that may affect your fireplace. For example, in certain areas chimneys must often be under a certain height, or you might have to obtain a burn permit to use the finished product. Plus, the odds are that you can't or shouldn't install a gas line yourself, so a pro is going to be necessary at some point, anyway.