I have been playing since beta, I have never bought a single pack (other than the one for the Golden Mekkatorque, from which I got no decent cards) and I have:
Millhouse Manastorm
Nat Pagle
King Mukla
Tinkmaster Overspark
Harrison Jones
Sylvanas Windrunner
Archmage Antonidas
Al'Akir the Windlord
Alexstrasza
Deathwing
Grom Hellscream
Lord Jaraxxus
Onyxia
Ragnaros
Sylvanas Windrunner
Plus Mekkatorque, ETC, Loatheb and Maexxna
All I do is log in, do my dailies and maybe a game or two if I feel like it. Two of those legendaries I crafted with dust - and I only dust cards in excess of 2.
It just takes time to get a good collection. And the decks you buy are exactly the same as the decks you get for playing, RNG, so all paying does is speed up the process.