Depends, are you asking for the going price for just the nuke, or do you want to include all the bribes, payoffs, donations to retirement funds, transport costs, and other miscellania that goes along with finding a nuke and moving it to the place you want to use it?
How big a nuke do you want? A tacnuke (a few tens of kilotons) costs a great deal less than a big stratigic nuke (a few megatons).
If you just want one weath check, I'd go with Arpad's suggestion, and start with DC 60.
If you want a more detailed answer, I'll assume a current world, post-cold war. A couple of dozen DC 20-30 bribe checks to find the device, A DC 45 check to buy it, plus another set of a dozen or more DC30-40 checks to transport it to where ever they want to use it.
In a world where the cold war is still active, I'd reduce the inital bribes by 10, increase the cost to DC50 and reduce the transport costs by 5 to 10.
In a world where a hot war is ongoing (say Armageddon 2089) I'd make it even easier.
Contrast this with the effort to build one of their own. From scratch. Most of the information is publicly available from a good university library. Obtaining fissile material is relatively cheap (DC 10-20 or so), though it may take some time to refine. You probably can't get a device with a yeild over a few kilotons this way. But much cheaper than buying an existing one. And requires a little knowledge. See
The Manhattan Project for an example.
More references: *
The Curve of binding energy by John McPhee. An older book describing how easy it would be for terrorists to build their own bomb.
*
The Radioactive Boy Scout, an account of a young lad who builds his own atomic reactor. In his back yard.