Unless Purdy is the unanimous NFL MVP and wins the Super Bowl in February, I think he's more likely to follow the path of another young, well-grounded QB who wasn't drafted in the first round but experienced enormous early success... and then got a new deal that pays a huge amount of money but also fits within the construct of the team's long-term payroll plans.
In real terms, Hurts got $179.3 million guaranteed over four years, an average of $44.8 million. That's the number I'd circle for Purdy and the 49ers when the negotiations start next year.