100$ million for J-20 is fantastic, J-20 is a heavy fighter so it is the equivalent to the F-22 not the F-35, the J-31 is the answer to the F-35.
the price of the F-22 is like 350$ million, even if they continued producing it in large numbers and somehow brought the price down to 200$ million that would still be double the price of the J-20. since the F-35 costs like 80$ million or so, the J-31 may end up costing the same as your average 4th gen fighter.
a while ago I was reading some articles/leaks on CJDBY that said the developers of the J-31/J-35 are now working mainly to (or already did) reduce the weight and cost of the J-31 while still keeping the same capabilities.