Filament Cost vs. Quality? I'm a 3D printing newbie, and I bought some Hatchbox PLA to start with since it is recommended by the community it seems.  

Is there really a steep quality drop off if you go for brands that are 50% less than Hatchbox?  What does that quality drop off look like?
In addition to @TecTec3's answer:
Parameters that define filament quality:

Diameter variance: If there is too much fluctuation in diameter, the filament might get stuck.
Material Quality: I had really expensive filament that was brittle and did tend to snap when it was extruded. Other filament just didn't melt but started burning up and regulary clogging my extruder.

My opinion:
I'd say you shouldn't go with too cheap filament and stick with one that works out best for you. If you have issues try another brand. It's impossible to tell quality just from the price.