There are a lot more factors to consider like the design of the bomb, how it's targeted, what the target is, atmospheric conditions and weather (which have a huge effect on fallout). A lot of that is not going to be in the public literature.
You can get a decent guesstimate by using a preset bomb in Nukemap (check out especially the advanced options):
https://nuclearsecrecy.com/nukemap/
You can also check out the definitive book on weapons effects, "The Effects of Nuclear Weapons" by Glasstone though I don't believe it discusses salted weapons.
Yeah, the ideal thing would be something like nukemap, but adjusted for the salted aspect. This question is at least partially the result of a conversation in which someone claimed that it would be possible to create a single bomb that makes an area as large as california, or maybe even half of the U.S., unlivable, which would be much larger than even the Tsar bomb on nukemap.
Seems like this comment was the best one, though overall I had hoped to get some more answers. But I will stick with my promise and am happy to PayPal you $50 to whatever address works best for you (or Venmo, or whatever else works for you).
As the explosion of the cobalt bomb will be very powerful, probably, around 10 gigaton, it will put almost all the cobalt in the upper levels of atmosphere, may be even in space, from where it will fall all over the earth. So, there will be no localised fallout, but it will slowly falling from sky all over the world for months. For example, large volcanic eruptions were able to cover all sky for months with soot. (There will be obvious local effects too, for the first thousand km.)
Check also this link: https://www.scribd.com/doc/234366347/Doomsday-Men-Cobalt-Bomb-from-the-book-of-P-D-Smith
and https://www.scribd.com/document/16563514/Herman-Khan-On-Doomsday-machine
Note, that 510 tons is only amount of cobalt-60 which appeared in the explosion, but as cross-section is not very high, actually one needs to put many thousand tons of raw cobalt in the bomb. In the links above is said that actual weight of the bomb will be the same as battleship Missuri, that is 20 000 tons. (ITER weights 23 000 tons, typical nuclear power reactor around 1000 tons).
Cobalt bomb may include not only cobalt-60, but other isotopes like gold, strontium or polonium, for higher killing "efficiency", as they have different half-life and different biological effects.
Also, the biggest effect from the cobalt bomb will be not acute radiation (as in the novel "On the beach"), but food chains poisoning, as some isotopes tend to accumulate in food chains, like strontium.
Some thoughts: At 510 tons, and the price of cobalt from the "Prices of elements and their compounds" Wikipedia page, the cost of of a cobalt bomb of that size is strongly lower-bounded by $15 million dollars, which is at least economically feasible.
More thoughts: The U.S. cobalt reserves appear to be around 23 thousand tons, which makes 510 tons feasible but not easy.
One can't convert all cobalt in the bomb into cobalt-60 as cross-section is not very high. I read estimates that the total weight should be around 20 000 tons, see more in the chapter of the book "Doomsday men" by Smith.
I think that actual price will be closer to other nuclear projects ITER or LHC, and be in around 10 billions dollars, but it is still affordable for major nuclear powers..
Cobalt bombs are the currently most effective way I know of to make large areas of land unlivable for a long period of time. I don't actually know the fallout area of such a bomb. The Wikipedia article only has the following mildly helpful section:
Theoretically, a device containing 510 metric tons of Co-59 can spread 1 g of the material to each square km of the Earth's surface (510,000,000 km2). If one assumes that all of the material is converted to Co-60 at 100 percent efficiency and if it is spread evenly across the Earth's surface, it is possible for a single bomb to kill every person on Earth. However, in fact, complete 100% conversion into Co-60 is unlikely; a 1957 British experiment at Maralinga showed that Co-59's neutron absorption ability was much lower than predicted, resulting in a very limited formation of Co-60 isotope in practice.
I don't know whether a single bomb that efficiently distributes 510 tons of material is remotely feasible, and the section about Co-60 conversion also suggests that the actual effectiveness of such a bomb would be far below this theoretical maximum.
Does there exist any existing literature that estimates the practical fallout area of a cobalt bomb? Or can we just do a simple Fermi to estimate things ourselves?
I am interested in this question because it seems relevant to estimate the worst case scenario of a nuclear war, and the likelihood of civilization recovering after such an event.
I will award $50 to the best answer by the end of next week, based on my gut sense, and I might pay out more if someone gives a particularly exceptional answer.