If you're going this route you're going to need to scale other things to real life cost too. That's not exactly balanced.
If you think nukes are SO OVERPOWERING, try being an immortal or deity game consistently using them and see what happens. You have to build the manhattan project (or in rare cases the AI might), hold off the UN resolution (requires defiance or to control the UN...), and build enough that they make a difference. And if you're too backwards, the targets will get SDI before you launch, making them very expensive to land consistently...even with tacs only 5/8 or so get through.
The game already has diplo penalties for it (kind of silly depending on situation), and already ties it to global warming (completely ridiculous on a lot of levels). What more do you want? Notice that every time you fire one, your power drops, so to compensate that you need to build conventional forces or you won't get capitulation...and war weariness might be a problem too.
They're balanced pretty well. If you have survived that long, you deserve at least a CHANCE. Don't forget that the AI *will* build them and launch them back unless you take steps to prevent that...
And of course against humans, they can be a lot more creative with ways to derail nukes or use them back...if the game isn't functionally over by then that is.