How much would you pay for a guest post?

That depends on too many factors in fact,
you can't rely only on DA/DR or any other metric,

ask for the domain and analyse it before purchasing,
check if the domain relevant to your niche or not
check the tier 1 and some of the tier 2 links of this domain
check the quality of the content along with the whole domain,
check the link structure (optional) to check if the guest posts have some internal links pointing to them or not (even by category pages),
check for which keywords are the website is mainly ranking for, how competitive are these keywords,
how much traffic is it getting,
how old is the website,
ask if you can make tier 2 links pointed to the post

and anything else SEO related (you can check most of these in a couple of minutes)
then you can decide if such a domain is really worth it or not

but don't rely too much on guest posts for ranking, you must use it for diversity more,
you can simply get PBN links that will rank even better for a cheaper price (quality ones)
 
I've one tech blog whose DA is 25 and I usually charge $20-$25
 
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