There are scattered groves of black locust trees here, and a lot of people use them for posts. I have plenty of honey locusts, the ones with thorns, and I'd gladly trade for black locusts. I do have a bunch of cedar, and that's what I use. The more red in them, the longer they last. It's said that the best cedar posts come from the poorest soil.
I don't know what 20 inchers cost, but 20 foot, 5 rail, 4' high are $130. and 14 foot gates are $125. We had a fence guy build a little set of pens on the south place, and an alley and sorting pen at the home place outta them. He used drill pipe for driven line posts and 5 inch pipe posts in concrete for the corners and gate posts. they turned out nice.... I'll do the north place with the extra panels and gates we bought.
the south pens
you can see the pile of old wood from the dilapidated wood corrals I tore out this last winter to the right thru the gate.... I'll burn the pile next winter when we get some snow cover.