I think they sell all the drugs first and then announce the total profits. Yes, police of course know how much drugs are wort in the street.
I do have a HUGE problem with these announcements however: drugs are supposed to be something evil and dangerous, which is why they are illegal in the first place. But saying things like “a kilo of heroin worth $200,000” just makes people think “wow, I wish I had 10 kilos of that then!”. I remember that’s what I thought when I first heard this as a child. I didn’t think of how many people would overdose on 10 kilos, or how many lives would be destroyed, what misery these drugs could bring and how they indirectly increase criminality. All I was thinking was how cool it is to be a drug-dealer and making all that money.
If the media want to really help in the war against drugs, they should never fail to remind us of how bad drugs are for society (and not just personal health), and not how valuable they are to their traders.