Based on the results of auctions this year in Spain, England, and Korea, the 3.7-4.2 GHz band would go for $10B-$25B. This was little used until 64 antenna Massive MIMO has suddenly made it very valuable. Antennas in higher frequencies can be smaller, and tens of thousands of 64/128 antenna rigs have shipped. The first results are customer speeds in the hundreds of megabits, confirming the trials and models. There's enough spectrum involved that all the telcos could double their spectrum holding.
In Britain, 150 MHz of spectrum went for 1.05B pounds, about US$1.35B. At that rate, 500 MHz would be $3.3B. The U.S. has almost five times Britain's population, so ~US$16B. None of the four UK carriers got more than 50 MHz, less than ideal. Bidding was strong.
In Korea, 280 MHz went for $2.7B. Adjusting for the U.S. population, that would be ~US$30B.
In Spain, 320 MHz went for US$511M. Spain had only three bidders and enough spectrum for all to get 90-100 MHz, the ideal for the band. So there was no competitive bidding and they got it cheap.
The FCC is considering skipping an auction for the 3.7-4.2 GHz bands, key for 5G. The market cap of Intelsat, a licensee, has gone up from $500M to $3B, just on the hope Pai and Reilly will let this go through.