A request which is even more fraught with Constitutional issues, not the least of which is the fourth amendment's limit on eminent domain (the Just Compensation clause) which requires fair market value be offered for this. Will Congress approve a single legislative action which will cost a trillion dollars? And which would make that trillion dollar investment worthless in less than six months?
Apple made $234B in 2015 in revenues, most of which was from iOS products. They don't break out the value of the software from the hardware but I would expect them to claim quite convincingly that half of the value is in the software. So assign a yearly revenue of $100B. Apply a reasonable market-derived value per revenue factor of about 10 and you end up with $1 Trillion being the cost of this little endeavor. That is about two years' worth of defense spending, by the way.
Coming from market value, the market cap of Apple could convincingly be claimed as at least 75% based on the viability of iOS. Which means that 0.75 x $567B or about $425.5B would be a market-based valuation of iOS, very conservatively.
Between the two methods of valuation, we have a $0.4-1.0 Trillion valuation at the low end, and up to $2.3 Trillion on the higher end (well, much much more, like $25Trillion, if you apply the EV or RV ratios to iOS instead of the lower Apple EV ratio).
If they did this, Apple's business would plummet. Nationalizing iOS would force the vast majority of customers to look to alternative mobile OS products. Apple obviously loses here, but so does the US economy. Android would obviously be the big winner, but then the DoJ needs to float the idea of declaring eminent domain on Android too, maybe Windows next, at which point we end up with non-US ventures like Tizen owning the market and the DoJ with at least $3 Trillion invested in worthless software.
Yeah, that is an empty threat from Comey and he knows it. As sensible as the Florida sheriff threatening to arrest Tim Cook because Apple has filed a motion to vacate the order.