Replying to Pressure Treated Wood

So i'm building a basic pvc rail and a ladder-style portable drop in, and I was wondering how unlegit it would be if i used standard 2x4's instead of using pressure treated wood. If I was to use the non-pressure treated stuff, I could get most of it for free, so I'd like to use that if it would last for a while, but I have heard a lot of conflicting information about how the normal stuff works as opposed to pressure treated wood. Also, how much does pressure treated wood typically go for in a home depot or something like that? I can't seem to find any prices online.

Thanks NS