I bought Hearthfire and Dawnguard on sale, but have not bought Dragonborn yet.
Hearthfire costs less, I think, because it is not a full-fledged quest-driven expansion. It allows you to get married, adopt a couple of orphans, and to purchase land, design and build a home for yourself in the countryside rather than in the cities. I've enjoyed working with Hearthfire. It was fun selecting which wings to my house I wanted, seeing it go up as I chopped the wood and built from the foundation up, chose my furnishings. EDIT: you don't have to get married or have kids to build the house. Likewise, you don't have to build the house if you just want to get married. All choices.
Dawnguard is about vampires. You can choose to side with the vamps or else become a hunter. If you install this mod, there will be vampire attacks in the cities that I don't think happen in the base game. Not sure on that because I had the expansion installed when I first started playing. I do have a mod also installed called When Vampires Attack that mainly scripts the villagers to run for cover when a vampire hits the town. I don't like to see my NPCs get hurt. Only the town guards remain to fight the vamps.
You do not have to be high level for Hearthfire or Dawnguard. Probably not Dragonborn either, since I heard that with Dragonborn you can learn new shouts. That would be useful during regular game play. Also, the cost of the base game has come down quite a bit recently, so that's the reason the expansions are closer in cost to it.
EDIT: Ok, I have to say a couple of other things about Hearthfire. You can have a Steward build most of the house for you. However, my Blacksmithing skill went way up while building my house. So if you're into making high level armor and weapons and need your blacksmithing skill to increase in order to get the perks to do that, building a house will help. In fact, you can build three houses with Hearthfire. So the expansion is not just about domestic bless.
I like living in the country rather than the city too. Beautiful scenery. (Says the woman living in the woods of Alaska!)