In my opinion:
Best bass for cheap:
Skullcandy Ink'd
Best accuracy for cheap:
JVC Marshmallow
Best bang for your buck:
Crossroads MylarOne X3 (Unfortunately this particular model looks to be discontinued, but Crossroads has since introduced the Bijou and Quattro. I just haven't had any experience with either of the new models).
Best HOLY CRAP! price I found while looking for the X3's:
Ultimate Ears Super.Fi 4 for $27!
Also, Etymotic ER6i's and Jays J-Jays are pretty affordable. I've never used either of them, but I've heard they're pretty good. Personally, I want a pair of Super.Fi 5's. Been drooling over them for a while.