From: http://tappedout.net/mtg-decks/100-affinity/

$100 budget version of one of modern's most played deck.

Sideboard is not included because it is entirely dependent on your meta and should be worked on in collaboration with others.

Key things to know about Affinity:

Mox Opal is the reason this deck can perform so well. Without it, this deck is not nearly explosive enough. However, this is a good place to start if you wish to upgrade into the full version overtime.

Ornithopter, Darksteel Citadel, and Ensoul Artifact and the unsung heroes of this deck. Getting a 5/5 flyer or indestructible can win the game easily.

Cranial Plating is the all-star of this deck. Put it on a flyer and swing for massive amounts of damage.

Also, this deck is very very tune-able. This is a template. The fine tuning is done by you.