Some shops offer services, such as a year's worth of tune-ups, that in the end will save you more money than a discount on the price. Your shop may offer other services, too, like a layaway plan. Some people buy bicycles with only the present in mind. If money comes easily to you, that's fine. But if shelling out a few hundred dollars isn't a regular occurrence for you, you should probably think about the future, as well.
Many people buy bikes with unrealistic plans.
Be realistic about your cycling goals and what you really need to reach them. If the price sounds too good to be true, it probably is. It may have two wheels, but is in no way even close to a bike shop bicycle. What additional cost usually buys you, though, is durability. As you go up in price, the materials and manufacturing methods usually reflect great improvements, making for bikes which will perform well for years instead of months.