I don’t think A Bombs are practical for this game. The scale is too big, the territories are just too big. In some cases, a single territory is a whole country (Japan for example). An Atomic Bomb destroys a city, not a whole country.
Now, if you could scale down the A Bomb’s effects to include 1 facility or 1 military unit, then maybe it would be feasable. I like the idea of an atomic hit removing an air base, naval base or Minor IC and reducing a Major IC to a Minor IC. It could also be used tactically against military units, but you would have to limit it to just 1 unit per hit. Because of the scale of this game, each unit we use represents a vast number of actual troops (1 inf = 1 inf division, 1 tank = 1 armor division, 1 fighter = 1 air wing, etc) An atomic bomb could destroy a division of infantry or an air wing of fighters, but not several divisions or air wings.
I think atomic bombs need to be like a tech development that you have to buy research dice for. In fact, I don’t think they should be available in the first round, or even the first few rounds. 10 rounds is a bit much, I think a lot of games don’t even last that long. I think atomic research should be a sort of tree, broken up into levels that you have to attain to move on to the next level. Perhaps 3-4 levels, the final level is where you would have an actual atomic bomb that you could use. So the earliest possible round using an A bomb would be round 3 or 4, depending on how many levels of atomic research you put in this particular tech. Odds are, your research may fail some rounds and other rounds you may simply not be able to roll for tech, so it would probably still be later in the game before anyone would be able to use an Atomic Bomb.
As for cost, that would depend on how easy you make it to be a successful Atomic strike. If it’s somewhat easy, then purchasing an A-bomb should be expensive. If it’s pretty hard, perhaps the bombs could be cheaper.
For Example: An Atomic Bomb strike is successful if you roll 5 or less. A roll of 6 is a dud bomb. So, A-bombs should cost at LEAST 20 IPCs or more.
Another Example: An Atomic Bomb strike is successful if you roll 3 or less. So, A-Bombs should cost 10-15 IPCs each.
Another Example: An Atomic Bomb strike is successful if you roll 2 or less. So, A-Bombs should cost less than 10 IPCs each.
Another idea would be that a player wanting to use Atomic Bombs would not only have to develop the Atomic Bomb tech (perhaps only 2 levels or even just 1) then you also have to get Long Range Aircraft and Heavy Bomber techs to go with it. This would represent the B-29 Superfortresses that were used to deliver the real Atomic Bombs. Since they flew at such a higher altitude when delivery the Atomic Bombs, I would say Atomic Bombers are immune to interceptors and AA Guns.