the value of guest posts comes from 3 things IMO:
- the size / popularity of the brand of the site where you guest post (a big, popular brand will count towards the trust that google puts on the links coming from that brand, and this is a good site to get backlinks from in the form of guest posts even if the initial SEO value / traffic are minimal in the beginning. So, a good chunk of money, say few $100s up to a $1000s, is justified in this case)
- the type of link(s) - d0-follow vs nofollow - in the guest post (d-0foll-w links are obviously more valuable, so they'll cost more)
- the traffic amount and traffic value (personally, I don't understand why the traffic value is so important but a lot of BHW members that I trust consider it to be important, so there is me mentioning it... anyway, the more traffic and the higher value this traffic has the better it will, hopefully, be for your guest post once the interlinking has been done and your guest post starts ranking and send you organic traffic. Not to mention that, over time, your guest will acquire link juice if the owner has a good site structure in place and / or builds links to your guest post thus increasing its authority and thus an initial higher price being justified as well)
In general, the size of the site's brand is what will eat up most of your money, while the link type and the keywords + traffic that the site ranks for / gets could mean a few more $100s. There's been a list of prices shared here on BHW a few months ago of a SEO agency that was charging $30,000 for a backlink from Cosmopolitan, and I dare say that the brand itself is what would eat up most of the cost of that backlink. Unfortunately, I don't remember what type of backlink that was (it could have been a homepage link for all we know), but even so, it speaks to the amount of money you can expect to pay for a link on a trusted site.
But if you're dealing with sites that only have few 1000s keywords ranking in google and / or are only getting a few 1000s visitors a month I think paying $50-150 for such a link is ok