Charon, you should be a weed-defense lawyer 
But seriously, no matter how it's rationalized, it's a lot of weed - and a 1/4 pound is no baggie - it's a BRICK.
Really, it isn't that much. Wouldn't raise my eyebrows in the slightest.
It's mid level dealer tonnage... ie: a dealer that sells to OTHER dealers and never sees end users.
At least in my area, that's not true... But I think from the rest of your post, I see why you're thinking that...
Most street dealers buy (more accurately get "fronted") an ounce at a time - and most end users buy a GRAM at a time (1/28th of an oz). A gram will get you plenty baked for an evening. Most drug users do not buy a month's supply at a time. Hell most don't look past the next 20 mins.
See, this strikes me as more of a city/poorer economic area blackmarket than what exists in more suburban/wealthy areas. The eighth is the more commonly purchased amount around here. If someone's buying a gram, they're getting ripped off and they know it. You want to team up with friends to buy in bulk to save money.
Now in the cities, of course, you can still find guys selling Nicks and Dimes.
ie: she had enough stuff on her to make 400+ people go to Dairy Queen asking for triple cheeseburgers with sprinkle-butterfinger-blizzards.
Another way of putting it is 400+ people can get stoned on ONE day, ONCE. Which puts it in a much less impressive perspective.
So I have my doubts... if she was so smalltime and innocent, how did she even show up on anyone's radar (not once, but twice)? And why was she used on such a high profile/dangerous sting ?
I was distracted while watching, but wasn't she arrested the first time after her car was pulled over? And then the second time on an anonymous tip? Well, college kids drive like jerks, and this girl was attractive. It's not too difficult to imagine that she po'd another girl over a boy. Women can be very vindictive.
As for why she was used on such a high profile/dangerous sting... Well, the Chief made it pretty clear that he's not the brightest bulb.