A mailbox post will cost you close to 30 bucks at the store, considerably less if you build it yourself.  The ideal situation is to have a mailbox needy neighbor who will gladly pay 20-25 bucks for their own mailbox post which covers your material cost and buys some beer to boot.  (3) 4 x 4 post will easily make two mailbox post.

Cut a foot off an 8 foot post, mitering the ends at 45 degrees, this makes the support for the arm.  Do this for each main post section.  The spare post will provide the arms, each cut at 32 inches from point to point.  The miters are opposing as a decorative detail and water shedding.

The lap joints are cut in the main post half of whatever the post measures, in my case 1 3/4 inches deep for a width of 3 1/2 inches or whatever is snug.  This is also done in the arm as shown in the photo above.

I joined the main post and arm with some carriage bolts and attached the angled support with some lag bolts.  A board was cut that just fit the recessed area of the mailbox and it was attached to the arm with drywall screws, the mailbox was put in place and attached with screws.  Works like a champ!

The mailbox went on the post AFTER the post was in the ground .  In this case, the height was being dictated by the existing boxes so all would be the same, making it easier on the carrier to deliver.  The post office DOES have regulations governing mailboxes and I suggest you Google for them.

Knowing that this is not a one shot deal and will need to be done again whenever the post finally rots or the termites eat it through, I did NOT use concrete.  Putting concrete in will just make it harder on me next time.  I DID tamp the dirt well using a scrap 2 x 4 and a small sledge hammer.  The post is sitting pretty solid.  Have fun with this and YES, it could be done for profit if you have enough demand.