While we all know drug price figures given by law enforcement are notoriously overestimated, I don't doubt this figure too much. Remember, the police are considering every ecstasy tablet to be worth $30, every OC80 to be $80, every amount of marijuana to be $60/3.5g, every hit of acid to be $15, every gram of coke $100.
A kilo of coke becomes a $100,000 value this way, a jar of 1000 ecstasy pills becomes $30,000, 10lbs of schwag becomes $78,000, and a bible of acid blotter becomes $150,000.
Since these were arrests made on the highway, and the police mentioned marijuana as the primary drug found, I imagine a lot of the arrested got caught with such quantities of marijuana in their car that the police could smell it when they stopped them.
Look at this report from a bust near Bonnaroo last year:
http://hightimes.com/news/ht_admin/4386
637 hits of LSD became $11,000 worth! $17.28 per hit is the way they calculated it.