Well it usually give up to 35-40% off in august-september somewhere for 10.0 - that would be as 10.5 is about to happend.
But it varies depending how old version you upgrade from.
And if you buy then - and wait to activate license until oct-nov - you get a grace period upgrade for free to 10.5.
Last and this year - it seems new release is november rather than december for years before that.
That is if being gready.
But I’d say that 20% of new price to major version upgrade, and 10% if minor version upgrade - if keep upgrading - is pretty decent.
It used to be much worse.
So if you want it - just get it.