I have the previous version of Har-Bal (an update was released a few months ago) and it's an excellent home mastering solution. Not only is it easy to use, it does exactly what the OP wants to do. You can load a professionally mastered song and either match it or use it as a reference to get an idea of how to balance your songs. It also has dynamics processing (now) and even loudness matching, which helps with multiple tracks with varying volumes. With that and Ozone, you could do a whole lot of mastering and save the bucks.

IMO, while it's certainly not the best idea to master your own tracks, it's a rediculous waste of money to pay someone if you're a home hobbyist and not planning some kind of pro career. Since you have the tools, why not give it a try? I think it's half the fun. Just make sure you take a few weeks off from the mixing stage to start mastering.