How much to charge for sponsored posts and ads?

I run a investment related news outlet that publishes news and reviews.

The site is currently receiving about 20,000 visitors/month, 99% of them being investors.

I want to reach out to relevant companies, but my problem is that i need to find out how much is "normal" to charge for sponsored articles/ads.

I have currently set up the following price list:

PROMOTION
$50 - Finished press release written by your team
$75 - Sponsored article of 600+ words
$100 - Sponsored article of 1000+ words

Ads - Price not set yet.

I remember reading somewhere that a good way to find out how much you should charge for ads is to use Google Adsense as a basis. I can't remember pretty well, but it probably had something to with getting the amount of money you make per month with Adsense and multiplying it by your page RPM.
So if you make $40 per month, and your page RPM is $1.20, then 40 x 1.20 = $48 is the amount you should charge.
By the way, if those are still your current prices, I'm interested in doing business with you. I'll PM you.

EDIT: I tried to PM you, but couldn't. Please get in touch with me.