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.

Check out http://www.wolgemuth-auction.com/Lumber.htm

I'm not affiliated but have attended and bought from them many times.
Doors go for $1 quite often when no one else is bidding for it. If you
do end up with a door that is not pre-hung its really not too big a
deal to hang it. let me know if you go, I may be there and I'll say
hello and guide you through how this auction works. Bring a truck and a
friend there are lots of good deals to be had. (Cabinets, plumbing,
electrical, sometimes tools, lumber sheet goods, tile, windows, doors
and some just regular junk)

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.