Use
https://tld-list.com/,
Sort the domains by renewal price and you'll get the cheapest long-term domains,
There are other similar websites for domains price listing and they usually don't come up with bulk prices,
Keep in mind, a PBN with a ton of fresh domains is not as useful, you'll probably end up investing more than if you're investing in auction domains,
The setup, content, design, reg dates, etc... to set everything properly will rise the overall network costs, it would be better to get fewer authority ones