My players are trying to get their hands on some powerful explosives in Stars Without Number. Enough to wreck a car or two. The planet they're on has fairly loose laws, but the thing they're trying to get is definitely illegal. Taking the illegality into account, how much should I charge for the explosives?
First of all let's define the cost for the explosive as if it was legal. Since I can't find explosive I'll start from the next best thing, grenades. Grenades cost 25 credit each, deal 2d6 in a 5 meter range. You say that the kind of explosive your player are after is supposed to wreck a car. A (ground)car has 30 hp. We need about 8d6 (28 damage average) to destroy it, which would be at least 4 grenades. Since a single charge of explosive is expected to explode all together, not in 4 separate explosions, we should charge more than 4 grenades.
Thus I would set our explosive cost to 150 credit (I can see it costing up to 200), dealing 8d6 and perhaps a bit more range on the explosion. Of course our charge can't be thrown since it became heavier than a single grenade, so it must be placed and detonated remotely. The detonator could be included or even be a onetime purchase.
The Black Market
Now we have to sell it. As you pointed out, the price should be higher. I usually multiply the price by 1.5 if many people know where and when to meet the dealer (it's an open secret more or less). On the other hand I hardly make my players pay more than 4 times the original price if they buy from more discreet sellers or if the character needs it on short notice.
We are now ranging from 225 to 600 credits, but we are not finished yet.
Finding The Dealer
Your player have to find the person who deals in explosives. Here things may swing a lot based on your table preference. If your group enjoys role-playing interaction and wants to expand their connection in the city, they may want to bribe someone or offer a favor (read: new adventure hook!) to get to the right person.
On the other side of the spectrum if your player sees a city as a big shop where they sell and buy stuff, just tell them to roll on Culture/Criminal and tell them how much it costs and that it took a day to find the right guy.
Wrapping Things Up
You may make them pay between 150 and 600 credits (based on group budget) for the explosives, but getting to the dealer may cost extra resources (time or more credits for example).
I never really handled a situation like that. But I guess I would imagine the kind of trouble and risk the seller had to go to obtain it in first place, and the risk of selling it to anyone (he might be selling to an undercovered cop).
I don't know what are the special properties of this illegal explosive (small, but extremly powerfull, made of certain components for a reason...), but that also would add some price on it.
Let's say, a common explosive would cost 100G, to make things easy.
Depending on those stuff, I would make it 5 times more expensive, or depending on the rarity of components, 7 times or even 10. That can also be less expensive, as you said, the trouble to get the explosive might not be too much, since the planet has loose laws. Remember that the players can always negotiate.
On the other hand, is the explosive needed for the story? Is there another way to accomplish the mission? If not, you can't make it too expensive that they can never buy it. Keep that in mind.
Hope it helps!