Low cost champs are often just as viable as high cost. Pretty much the only difference is their age: Older champs are anywhere between 450-4800 IP whereas all champs within the last year or so cost 6300.
None of the low cost champs are useless, as they can all be played successfully in their intended roles. Poppy is difficult to make good use of unless you know how to use her, but characters like Annie, Ashe, and Nunu are terrific without a steep learning curve.
Garen is actually considered one of the most powerful, if not THE most powerful, top lane in the game right now.
If I were a woman, I would spend my entire day kissing other women. I don't understand gay men or straight women.-- Jim, The Newsroom
Cheap champs you should definitely buy: Amumu Garen Morgana Nunu Ryze Taric
All of those are cheap champs that see top-level play.
You should give serious thought to also getting Annie and Kayle, maybe Cho'gath or Malphite too. Get Tristana and Alistar for free with the Facebook and Youtube follower thingies.
I wholeheartedly agree with all of these champions, with the possible exception of Annie. They're all incredibly useful, and even Annie can be scary. She can just be shut down pretty hard by a long-range champion who knows to stay away when she's got a stun nearly up.