GK - There is as yet no mention of whether the Navy wants to buy 20 F-35Cs (FRP) in 2021. That was supposed (as of last year) to happen in 2020. By the way, the 12 F-35Cs in 2020 will cost $144m each versus $81m for each of the last batch of 26 F/A-18s. That's rather more than inflation.
In the 2020s at full rate the U.S. F-35 program will cost (if the budget trends for FY19-20 are correct) >$14 billion per year, including c. $700m just for upgrade R&D.
The full-rate unit procurement cost for the F-35B is $147m, incidentally, and don't forget that the partners will also be billed for upgrade R&D at upwards of $2m per aircraft per year. (That number may come down as more jets join the fleet.)
Also - since partner contributions boost that Block upgrade R&D above the $700m U.S. figure - it's interesting to note that two years' work on a five-year Block 4A/B program (to add some new weapons to the F-35 and fix a few issues here and there) is roughly equivalent to the entire JAS 39E R&D bill.