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@TheMarquis said, the traffic is one of the most important metrics when setting a price for your guest posts. But it's not the only metric and, depending on the type of client, not the most important one, either.
For example, casual SEOs / webmasters who don't know too much about SEO will be happy with a guest post on your site if that guest post brings them a few visitors a week because that traffic is - presumably - very relevant to their site and could convert into sales and if they make a sale a month out of that traffic by paying $50-60 for a guest post that will last forever, and which will grow in authority (PA / UR) over time... well, they're more than happy paying the $50-60 that you might charge them if your site only gets a few 100s of visitors a month. So, this type of client (casual / newbie SEO, or non-SEO savvy webmaster) will look at the rankings and traffic metrics of your site before deciding whether to buy or not. So, for such clients, asking for $400 for a guest post will most likely deter them from doing business with you, but the question is: how can you know what type of client is asking for a guest post? You can't really know this so you can charge them more, so you'll have to price your guest posts a bit less until your become popular enough to charge whatever you want
Now, for SEO-savvy clients (SEO agencies, PBN owners, and link builders / sellers in general) things are different. This type of client is mostly interested in the SEO value of your site, which ultimately comes down to the number and quality of the RDs pointing to your domain. Of course, this client will still check the traffic and ranking metrics of your site, but more like a precautionary measure than a crucial one because sites that don't rank and don't bring google traffic in are usually considered dead in the water and this does impact the quality of the links you're getting from such a site, even if the site has good DA / DR (say, 20 to 40, which is pretty decent).
So, this type of client (savvy SEO) will only check the traffic and rankings of your site just to make sure that you know what you're doing but they're expected to be charged upwards of $100s if your site has a lot of quality RDs pointing to it. They understand the SEO value of backlinks and are willing and - more importantly - capable of paying you good coin for a backlink from you. But again, how can you tell what type of client is contacting you so you can offer them a fair (for both parties) price? Tricky indeed....
So, what you could do instead, is price your guest post based on the number of RDs pointing at your site, at a $1-5 or higher ratio. So, for each RD (unique domain that is) that's pointing to your domain you could charge a minimum of $1 if the RD is decent (ie. not spammy in nature), but the price could go as high as the quality of the RDs that are pointing to your site. If your backlinking profile consists of only powerful brands like Forbes, Cosmopolitan, Bloomberg, JP Morgan, Entrepreneur, Wikipedia, NY Times, etc... basically only huge, authoritative, trustworthy sites, you could charge upwards of $10 for such a RD and it would be a fair price.
So, the price you'd charge for your guest post would be a combination of these:
- upwards of $1 for each RD that's pointing to your site, based on its quality;
- value and amount of the traffic that your site gets;
- (optionally) the niche you're in (some niches - the high paying, or extremely competitive ones - such as: medical, finance, business, education, travel, luxury cars / yachts, insurance, SEO, etc, would have higher costs associated with them than the less competitive / less paying niches such as: arts, hobbies - although some hobbies can be really high paying - memes, etc)
For example, let's say that your site has 80 RDs of decent quality that are pointing to your site, and that you get 3000 visitors a month from google and of those visitors 70% (so, 2100) are coming from tier 1 countries (tier 1 countries = countries with high paying potential, such as USA, UK, Canada, Australia, Germany, Netherlands, Switzerland) and that your site is in the "long distance learning" (so, education) niche.
Well, based on the above numbers you could easily charge $100 to a casual SEO / non-SEO savvy webmaster, and $200 or more to an SEO agency based on the assumption that the RDs pointing to your site are not big, popular, authoritative names, but not bookmarking sites or web 2.0 platforms, either. I assumed a $2 per RD ratio when I made this calculation, that's how I came up with the cost of a potential guest post.
Of course, the numbers that I came up with in this example are made up by me, so you shouldn't take them as benchmark for every guest post you sell. But they're still valid (as they're based on my SEO experience) as a starting point when setting up the prices.