It depends on how the PBN is intended. You can profit from just 1 post, as in a newspaper or a guest post service. Obviously the PBN is linking every single article to promoted sites, then it will end as a link farm.
But generally dedicated PBN are also based on a monthly payment as in a SAAS, obviously being more costly.
It depends a lot on the purpose of the buyer: diversity of linking sources for a well established source that want to pump or building up reputation.
If you are willing to push one long-tailed keyword article, then hard linking from just 1 source, its clearly an overkill. But if you have a brand new site and want to boost full authority, then it might be better to start off by renting some PBN homepage links.
But I don't clearly see what's the point of purchasing 1 or 2 links in any of the two options other than testing purposes, that could be seriously difficult with such low amount of links. For the first option, 10 or 20 links can cost no more than $50 per pack and in the second option, its easy to find a $50/mo rent package among sellers. And spending less than $50/mo in this kind of services, when $50 is the bare minimum to start CPA in Facebook, is barely peanuts in terms of SEO.
Most of my colleagues with very low budget, just prefer to go cheaper with manual quality links by spending their VA or their own time...