This isn't practical in all areas of the country, but you might consider comparing locust to cedar, especially if you are building anything yourself. I refuse to use pressure-treated wood on our farm, but I have found a fencing company which sells locust posts - we are using them for all of our horse fence and I buy blanks for the chicken run, garden, etc.
Locust is just about the most durable wood around after osage orange. Posts last twice as long as cedar ... anywhere from 30-50 years. If you happen to live in Appalachia or the Midwest, it may be freely available (you'll have to look around though - Home Depot won't have them); here in the NE, my fence company imports them from North Carolina.
Companies which can get these post in bulk offer GREAT prices. Here in New England, I buy from Springfield fence in VT ... a 6x6"x10'L post is ~$17, which is even less than the price for a similar PT post, and FAR less than a cedar post.
I know it's not for everyone, but I thought I would share. When I discovered locust earlier this year, it perfectly met my criteria and budget - I was ecstatic!