larf or larfing?

Not sure why you are asking this here, but here you go...

http://www.urbandictionary.com/define.php?term=larf

 

You probably want defenition 4 on that link then...

the shake from green weed after manicuring the buds.
That larf will make some righteous green butter.

I don't think it has anything to do with growing, just manicuring.

 

How much does this stuff usually cost? Do people smoke it? Or is it just for extracts and/or edibles?

 

this is really old, but my understanding is this is generally what happens when plants have some problem in flowering (one common example is too much N from veg nutes), which causes your flowers to be not so dense (usually looking more sativa-ish, but still more stretched out, light weak flowers). I guess this goes with the UD explanation, as you would end up with a lot more leaf in the trimming process.