what will you do if the bitcoin price rises by a factor of 100? your domains will cost 100 times what they did before.
I think you might be conflating two costs:
Cost #1 is the cost to get a bunch of domain transactions accepted.
I believe pent's proposal is based on additional difficulty, which will be independent of the bitcoin price.
In other words, if bitcoin difficulty is 100, then maybe you need to solve a difficulty 110 block to get your additional DIANNA data accepted by the other DIANNA nodes who are maintaining the key:value database.
If bitcoin difficulty rises to 1 million, then you need to solve a difficulty 1-million-and-ten block to get your data accepted.
The additional difficulty factor should be set by the DIANNA nodes based on their ability to process/store data. More thinking is needed about what that function looks like (it reminds me of the bitcoin fee algorithm; it's trying to solve a similar problem, preventing spam/abuse but allowing as many transactions as possible for the lowest possible price).
Then there's a completely separate Cost #2, which will be higher than cost #1, that is the payment that miners (aka registrars) will charge to handle domain transactions (and do the work of bundling them up, creating some sort of summary hash, and getting that hash into the bitcoin block chain). That's just a free market.
(I'm completely ignoring Cost #3, which is the "what if I want to purchase an existing domain" price, and is whatever the owner is willing to sell it for.)