Jayce is pretty annoying, best case scenario for nasus is that he looses out on a lot of cs. Worst case jayce gets fed and snowballs, also takes his tower early. Then you hope you win before the game drags on and he manages to farm up Q despite that.
Best case Nasus gets to level 9 and buys a chain vest+Scepter, rendering Jayce 100% unable to do damage to Nasus in any meaningful way. Worst case, Nasus gets to 9 and Jayce has like 15 kills or something crazy. --- If inflaton turns $3 into $0, then $6 turns into $3. -Orca