In Itch, with the PWYW model, you have no way to limit the minimum purchase price.
Under PWYW if a person donates 1 dollar, for Itch, that person has purchased your project and has full access to the project.
The "automatic" alternatives that I think could be valid
A)
Create a discount coupon and place it as a FILE with a minimum price within your PWYW project.
If someone donates less than the price of that file, they will own the project, but they will not be able to download that file and therefore will not be able to access the coupon. (People who have already purchased your project will be able to access the file if the purchase price is equal to or greater than the price you set for the file.)
Con: I think the coupon has a lifespan, but I'm not sure.
Also, if someone shares the coupon on the internet, anyone who copies the coupon URL will be able to use it.
B)
The other alternative that occurs to me, which I see as more feasible, is that you generate a second page with the PWYW project and leave the first one only for payment. You can hide it so that it is not indexed and place a link on your PWYW page explaining that if someone wants to donate X amount to you, they can do so by purchasing the old version so that they can continue buying the new versions at a discount.