Pre fab aluminium can be cheaper than wood if you use a pre made kit that doesn’t allow for customization. If you need a specific size or shape and a pre made aluminium kit won’t work, customizing aluminum to fit your needs can get more expensive than using wood. Good wood, with proper treatment, will resist rot for a very long time. Some people love natural wood, others don’t care.

I don’t recommend doing it your self, much of the job is a two person effort, especially if you use larger wood. Hoisting a 20′ 4×12 frame member 12 feet up in the air is not easy by your self.

Also there are plenty of codes that need to be followed, not the least of which is how you mount the ledger to your house. Mess that up and the problems are immense.

Another thing, your cost will vary depending on if you get a licensed contractor who gets a permit, or a free lance carpenter guy who doesn’t get a permit. Its your choice, but I think the risk of potentially having to tear it all apart and start over outweighs any potential savings.