Tile improvements don't cost anything unless they're roads or railroads. The ingame text is badly worded.However, I'd like to see a "remove improvement" option for workers (if this is possible) because it would grant a much more lucid map (in time) after take over an enemies lands. I think the AI builds improvements in every hex possible and I don't always want that. Start building anything and abort prevents the cost but not the visual flaw.