I think that the bomb upgrades should cost about 20 percent less. The small bomb does only 10 damage which is nothing compared to the tanks after mission 10. I had to keep on saving which caused me to have more trouble on harder missions(I couldn't get other upgrades). Once I got it, the bomb did 15 damage- still nothing! This is the problem with the bomb upgrades being very expensive. If they were cheaper we would have less trouble with missions while saving