Not all bike brands are created equal. There are a few ........ that dictate how relatively expensive a bicycle is.
Material: Some brands use lower end materials. You do not need a degree in chemical engineering or materials science to discern the difference between grades of steel for instance-- some are much heavier than other.
Build Quality: Some brands are known for building frames a certain way. Cannondale has traditionally been known for their very nice looking welds.
Where it was built: Contrary to common perception, most bicycles are no longer produced in the United States. While some boutique brands still build frames in small batches by hand, most production has moved to Taiwan. Many bicycle companies do not even produce frames any more but rather outsource production to independent factories or other companies.
ProTip: One thing to keep in mind when buying a bike used or from a store like Target, Walmart, or Costco is how much money you plan on spending on maintenance the moment you buy your bicycle. At retailers like these, it is not uncommon for the technician who is responsible for putting together pool furniture, bbq pits, and anything else they sell to also be responsible for building bicycles.