I read through the Black market items section last night (CRB 150, I think), but I'm still not sure about the cost of a restricted item. This much is clear:
Restricted items require a Streetwise check to locate, with the rarity modified by the chart ranging from Core Worlds to uncivilized planets. However, how much is the item once it's been found? Does it cost the listed price, or is the purchase price also modified by the adjusted rarity?
Part of my confusion stems from the trading rules. When you purchase black market items from one world, then attempt to sell them at another, the sales price is adjusted by the difference in rarity between the worlds, with a potential for up to 4X the value. If the value is increased due to rarity, it makes sense that purchasing follows the same rules, but the CRB doesn't go into this.
A corollary to the above, but the CRB goes into much better detail re: the selling side of things. There are clear rules on how much Streetwise and Negotiation effect the value of sold goods with successful checks pushing the resale value up to 75% of base value. However, nothing is said about how things work in reverse, when you're trying to purchase something from a merchant and want to haggle the price down.