My breton character just married alea the huntress (we're both werewolves) which house in skyrim is the cheapest to buy?

The house in Whiterun, Breezehome.

There are places you can get for free...if you do not mind getting your hands dirty.

As far as I know, it's a tie of sorts.

Breezehome: 5,000 gold, no matter what.
Honeyside: 8,000, but you can get it for as little as 5,000 (maybe 5,001) if that's all you have.

If you want Honeyside, just give all your gold except for 5,001 pieces to Aela.

As for the better choice... if you don't have Hearthfire or don't want kids, it won't matter. Pick the city you prefer. Note that if you buy a house in Riften, you can sleep in Jarrvusker when in Whiterun.

If you want to adopt, and like to do Alchemy: Honeyside allows a kid's bedroom AND an Alchemy lab. Breezehome makes you choose.

The abadoned house in Markarth, you can get for free if you finish the quest.

This is correct if I remember correctly.