The posts in your pics are iron, likely well casings. Steel pipe will be expensive! I'd guess that the owners of those posts know someone in well drilling, or scrapping.
There's a pasture near us with 6" steel posts all throughout and 1" cable as fencing. Apperently grandpa was in the crane drag line business, so had access to the materials.
We have pvc fence around our arena, but I'd never build a fence on it to pasture stock. There's vinyl that goes over wood posts to pretty them up, but is not the actual structure of the fence. Which you could do with pvc but it'll be expensive and dangerous. Pvc tends to explode into sharp fragments when struck, after being exposed to the sun for extended periods, even after say 10+ years not in the sun. It dries out and becomes brittle.
Best thing you could do for post, though looks may suffer, would be to find someone clearing land. Buy the locust, cedar, even pine logs from them. Super cheap, a lil more work for you but they'll be green, very cheap, and if ya get the right logs, last a long time.
Believe it or not, we just chipped about 1000' of 10" pine logs and close to the same of locust. If someone asked, they could a had it all for free!