Well, it's worth maximally 1/2 culture if we're just summing the number of yields, but theoretically, the border growth to instayield exchange rate gets worse as you claim more tiles, so its probably even worse than that. Early game, 1 point is a bigger impact than the monument -25% reduction to tile cost, but less than half a whole monument, and 1/3 of the free per city generation of Epona prior to the Celts UB. I don't think that's OP at all, especially when you consider arguments that early border growth is probably not a great thing because it prevents you from accruing the higher yield, later era per plot instayields. Though if we want to understand how border growth intrinsically converts to usable yields we'll have to model the tile claim yields under various conditions (i.e. just authority, authority + GoTE, authority + sovereignty + GoTE, Celts with Epona, and so on) with some culture / border growth point per turn game data over a full game to see if its too strong or weak. I suspect it will be a decent boon for authority/border growth strats especially on higher difficulties as it gives the option to purposefully drive your expansion rate in your expands to secure more yields (especially if you build one fort that can be worked by multiple cities). If it's too strong of an impact but we like the idea of forts making border growth, I think we could also pull on border growth yield and border growth rate levers a small amount to find a happy balance. We've been discussing in another thread the strangeness of the current border growth formula (-% reductions should be multiplicative not simple subtraction from 100 to avoid a forced floor), so it may be a reasonable opportunity to tweak the math there as well if we're fine with potentially destabilizing border growth for a version or two. The multiplicative proposal is a nerf to border growth cost reduction and border growth overall so that may help counter balance any additional yields we put on tiles.