Hello,
I was looking for the same thing, I'm no expert in CSS but this worked for me for the template 'Marta'
in Design -> custom CSS add the following code
.sold-out .sqs-money-native { display:none }
-> the price is now hidden for sold items (out of stock) on all pages displaying the products
I'd be happy to hear if this code is not reliable.
see the example on the website: https://www.erinharrison-art.com/shop
cheers
Vincent.