Just get the bike and don't worry about gas prices so much. The cost of the bike, helmet, leathers, insurance and maintenance already create a total cost that will be difficult to offset on the savings in gas alone. It might take years to do so, in fact.
A bike is more convenient to park, but just as big a pain in the ass as any other vehicle (and often less convenient because of how much less stuff, like groceries, you can haul). You get wet when it rains and nobody sees you coming, so your potential for having an accident is higher. It doesn't matter how good a rider you are, Mr. Murphy is your constant companion. It certainly makes you a better driver!
I've commute with a motorcycle just about every day for the last three years. I'm beginning to think that they are as impractical, if not more impractical, than owning a sports car. They're fun, but don't delude yourself. If you do buy one, get one that is 400-600cc, pay for it outright (i.e. go cheap) and take a rider safety class at a community college. If you buy a used bike, bring a friend with you (preferably one who knows something about motorcycles, but anyone who will talk you out of making a foolish purchase will do in a pinch).