With the massive upsurge in interest in Mobile Frame Zero, a lot of talk has been about getting the parts you need to make the mechs you need to play the game. Most of the talk revolves around using Bricklink and Pick-A-Brick to cherry-pick the exact parts you need. While this gets you the parts you need fast it also gives you only the parts you need and no more. You won't be left with spare parts to build Stations and scenery. I'm also a fan of having a stockpile of Lego parts for later use.
Here's the list I've compiled so far:
Hallowe'en Spiders polybag.
Atlantis Octopus polybag.
Atlantis Mini-Sub polybag.
Star Wars Brickmaster.
Star Wars 2011 Advent Calendar.
As you can see most of the sets are ploybags. These offer the most bang for your buck; lots of small pieces -- generally of the kind needed for mech building -- and reasonable prices.
I saw this one a couple of days ago, 40031 Bunny & Chick polybag. While there are some "enh" parts in the set, it has four Travis Bricks, two Headlight bricks, and some other good parts. It also has 52 pieces and retails for $4.99 USD, which means it falls into the $0.10 per part range which is right in the sweet spot. Also, it's a seasonal item so, if it stays past Easter it'll probably get marked down.
Another one that's good, but not in my usual price range for recommendation, is the 5767 Cool Cruiser. It has all the usual fiddly bits we love but, the thing that knocked my socks off was the two Trans-Clear Headlight bricks.