Its not that hard - First of all. Is the door you are looking at an exterior door? I haven't seen exterior doors at the orange store that were not pre-hung. Make sure it is and exterior door.
BTW - Can you wait a bit? if you are willing to wait and to drive to Lancaster County PA (Not too far from Baltimore, under 2 hours for sure) There is an auction Thursday October 13th in Leola PA. They will likely have your door for a great price.
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I bought an exterior slab door (not prehung) in wood, 6 true divided lights for $35. It probably cost me another $50 for the materials for the jambs and threshhold and hinges. Took a bit longer to install than prehung as well but it looks great.