If theres 20% plus increase in performance and i can seel my 2080ti and throw maybe £300/400 extra.. max... if needed then i might bite the bullet and upgrade. Anything under 20% and i think i'll stay with what i have at the moment.
The irony is that the value of Ampere is inversely-proportional to the resale value of your Turing card.
If Ampere is just another Turing generation, your 2080ti should hold a good portion of it's value. If Ampere goes the way of most of the generations *before* Turing, your 2080ti will lose a large chunk of its resale.