I would use a 6x6 steel painted post. Will be much cheaper than $600 and your double gate won't sag. If you do go with a wooden post, it should be braced like the attached image.

This post was edited by dkv-texas on Fri, Apr 26, 13 at 14:50

I think it comes down to the design of your fence and gate, but like dkv, I'd go with steel for a heavy gate. For light gates, such as pasture gates, we've use fir posts, but when we installed DS's entry gate we used steel powder-coated posts. The rest of his fencing is fir and electric braid.
Our gate also has steel posts, but we had brick columns built around each fence post, with light fixture on top.....a little more dressy than DS's.

Thanks for the input. This fence is more decorative than functional (it needs to work and open, of course, but I'm more interested in a good look). Therefore, I don't think steel would work. I'm looking for something wood.