I really hope they do. I'm not pre-ordering Fallout 4, and if I was, I would be doing it through Steam, so I wouldn't get the Perk Poster either way. If the poster was made available for purchase separately through Bethesda's store, I would gladly buy it.
Around $15 (+shipping, tax, ect) sounds reasonable, but I would be okay with paying up to $20 for it. $30 would be a bit too much.
EDIT - I changed the first option from "Yes, I would love to own it!" to "Yes, it should be available to everyone!" since a few people who pre-ordered anyway voted yes, just because they feel people should be able to buy it if they want to. Or because they pre-ordered digitally, and the poster only comes with a physical pre-order.