I think that special hardware to break RC4 would require 256 bytes of registers and only a few hundred control gates. Lets say 5000 transistors per "module". You can put several hundred modules on a chip. Each module can easily do one step in 5 ns. I havn't figured out what the attack would be (known plain text etc.) and hardware to handle that might be more. In mass production the marginal cost of such a chip might be $100. Perhaps trying one key requires 100 steps. I get the cost per key trial as follows: (100 $/chip)(100 steps/trial)(5 (module*ns)/step)/ ((10^9 ns/sec)(10^8 sec/(economic lifetime))(200 modules / chip)) 10^(2+2+.7 - (9+8+2.3)) $/keytrial= 10^(-15+.4) $/keytrial = 2.5*10^(-15) $/keytrial I compute the cost of breaking a 40 bit key as 2.5*10^(-3) $ or one quarter of a cent.