I've only spoiled myself on homes so far and finally bought the most expensive one. It's not really difficult to get either and isn't involved in the main story. But here's the little bit of information I know:
There are 5 purchasable houses according to early information.
I only know of two off hand:
Whiterun is opened up via main story
Solitude is gained by completing the first quest the Jarl there gives. She may not give it to you until you complete some quests from some of the important people in town whos names I can't remember.
Cost of house:
Approximate cost of upgrades:
I don't believe the others are gained through the main story either, but my knowledge of them is limited.
Though once Sunday hits, I won't be touching Skyrim for a while. Gotta play my Zelda.