First of all, street values of most drugs can be really volatile, depending on hiccups in the supply, competition between distribution networks, how much Knight Errant has been cracking down lately, etc. Also based on the quality of the drugs (or at least, the credibility or proof to help convince people that the quality is good)
I'd also think that would vary a LOT by how they sell it. If they know end users and can sell it to them, they'll get more than if they sell it to a dealer in the stuff, which would still be more than if they just sell it to a more general purpose contact.
As a GM thing, unless they want to play it out, to get more value from it, I'd wave my hands and say "Your fixer might know somebody who knows somebody who might be interested in moving the stuff. But it isn't her usual line of business and handling it at all carries its own risks, so she'll offer you X nuyen, that she'll pay once she's moved it up the line." ('X' being whatever feels like a nice bonus in your game, without warping things in ways you don't want)
If they do want to move it to someone willing to pay more, then they have to deal with the fact that they are no doubt cutting into someone else's business, and that someone won't be a fan of competition.
Another alternative is to keep the shipment as a sort of McGuffin. Their next 'job' is actually getting it to a customer, and their pay for that job doesn't come from a fixer or Johnson, but from the buyer.
