SUBJECTIVE buy priority of ALL DLCs for beginner players this is almost certain to vary depending on your gameplay preferences. Not just for the Lord, but for the special units that come with. *If you really like a specific faction, buy the Lord DLCs corresponding to that faction.
In terms of content for cost, I recommend picking up race DLCs over Lord DLCs. The Empire recently got a BIG update for the Mortal Empires campaign. Specifically, it allows access to the Empire, Bretonnia, Dwarfs, Greenskins, and Undead. This gives you access to the most amount of factions in the Mortal Empires campaign (which is sort of like Warhammer 1's campaign, but with the all regions and factions of both games). If you must only get one DLC's worth of Warhammer content, get Warhammer I for the most bang for your buck.
In terms of cost-efficiency, this does not account for sales newer content is less likely to go on big sales so it will change with every sale. This post is created a little after the release of the Hunter and the Beast DLC.
