Cost/Content ratio wise get Dragonborn: A new questline, new shouts, more armors, items and potions to make, an entirely new map to explore (well not technically brand new, just an updated version of a map in a previous game but still you get the idea), and new creatures to fight.
Also just for the heck of it get Dawnguard as well. You'll get a few more things that aren't in vanilla or Dragonborn (a new questline, arrow/bolt smithing, crossbows, new creatures, more caves/ruins to explore), but this one is totally up to you.