Price of PBN are growing proportionally to the maturity of the technique. Meaning that paying for low cost PBN won't drive you many results.
Niche are important, if you are about to buy, I would recommend you to ask the seller and if they sell generic PBN, then go for branding or naked url domains, which should always be a very high % of your linking profile (60-80%). Also you can go with some generic keywords, like here, or click here.
Obviously noone wants to pay a lot of money for a brand or generic anchor, but some powerful links from a PBN despite being this type could help you get authoritativeness, which may push any of your sites to page 1 promptly.
Good quality providers don't need to offer a good cost. They just are constantly searching from new fresh domains, which could be very costly. Paying +$3K for a 40+ DA/DR domain is not rare nowadays.
So obviously don't expect to get one link from that domain in a $30 for 50 posts BST.
Don't be fooled. PBN are like consumables, so the higher the quality the higher the cost, they are not assets that increase their value over the time.
No need to be competitive because nowadays I will say that at least 90% of the good quality domains are being brokered. 5 years ago, maybe only 30% were being brokered so there was greater variability in prices. When you pay, you pay for not having to
a) search for domains b) avoid the uncertainty of buying a shit domain for $1K+ c) having to maintain a couple dozen or hundred domains in dozens of hostings d) not having to build each page and make them look real = at least 50% of the real cost of most domains, which is the real only element that makes different one pbn from another (the time cost of building and maintaining, just like any other SEO Activity).
