I don't even need 1k stacks to take on a Jax as Nasus, 450 will do in most cases, especially with Jax's new squishier builds
Anyway, this must've been a really stupid Nasus you beat down, or if you're speaking from theory then let me break down some numbers:
Its now possible to get 40% CDR with
, assuming Nasus is farming competently these 3 items net to being 6650 gold and about 360 stacks around 20 minutes, cheaper than

which nets to 7200 gold
alone add up to 6300 gold. Or if you don't opt for a Triforce, then
add up to 5900, with a
, that's 6950 or a
its 7100
If you were jungling, then
or
adds up to 8750 gold, if you want to break apart the triforce pieces, then it'll add up at the cheapest with
at 6000 gold
Oh cool, finally a cheaper build than Nasus' right? Alright cool...but you have way less health and Nasus' HP pool and resistances with his items will allow him to shred apart Jax, 90 extra armor
Nasus gets 70 extra MR and 500 extra HP vs a Jax who is relying entirely on his nerfed to oblivion base stats and a full damage build, at most he'll get 250 more HP with his laning build with a Triforce
With 40% CDR I can almost PERMA-WITHER a Jax, with a rank 1 wither (with a STUPID nasus), that's a 9 second CD with a 5 second duration meaning 4 seconds Jax can viably fight back. With Rank 5 Wither (a SMART Nasus) this will be 7.6 second CD, 2.6 seconds Jax can viably fight back
BUT WAIT: Frozen heart reduces attack speed by 15% and stacks with Wither! Even without wither, when Jax (A champion who relies almost entirely on autoattacks) jumps on him, he immediately loses 15% of his attack speed. Even worse is he needs to keep attacking to get his own speed stacks
Additionally: the new jungle nets 34 less gold per clear and than before whereas lane minions net more gold, even worse for Jax: The items all got jacked up in price!. This means that while the jungle Jax build is cheaper, it's harder for him to get the gold for his items. He will NEED to get fed to get his items in time vs a farming Nasus and if he does this, he has less devourer stacks
For laning: Nasus has sustain, Jax doesn't. A stupid Nasus will let Jax bully and zone him, a competent Nasus will trade back and abuse his sustain advantage. The CS, and gold, should swing towards Nasus' favor
In other words: Jax vs Nasus...very strongly in Nasus' favor, the Jax must rely almost entirely on being fed and the Nasus being stupid/very behind to stand a chance