How much do you cost per blog post?

Hello everyone,
This is kinda method + I have some questions to ask.
The thing is I am not so good at blogging but I built a blog around a year ago to earn some money from adsense.
Now, it receive around 250,000 pageviews per month and I am earning a decent money from it from adsense.
From last two weeks, 2 people emailed me and asked they want to buy a blog post in my website.
Surprisingly, they bought a blog post in my website, and 1 paid $200 and 1 paid $110

Now I am also contacting few companies to buy blog post or links, but can you guys tell me how much should I cost?
Because I asked $200 to a company and they said they cant pay more than $20-30 and many of them didn;t even reply.
If you need more information, my blog is tech related, it has PR 2 and alexa is around 88k.

P.S : After the advertisers contacting me, I kinda have feeling that the blog can get me some more extra bucks

See its all about the niche of the site and the buyer intent, If your site is on the make money online niche expect a lot of revenue from blog posts, sponsorship's, guest posts, link sales, product reviews to come in automatically.

Is-there a website to know that, depending of the the niche sector ? (pharma, gaming,...)

its depending on your site Quality and the niche...so can't say the exact price without check your site...sorry