Never be shy to quote your price for the sponsored post. You have earned the privilage by working hard on your Blog and the sponsorers understand this.
Be confident of your quote and present it to them in a professional manner.
I think you should take your blog readership,subscribers and followers seriously when quoting a price instead of your Pagerank or Alexa Rank because you are putting your post in front of readers and not selling a link from your blog.
Another reason to quote high (reasonable) for a sponsored post is because you are attaching your brand with the company and the post stays on your blog forever,for a long time.
Enjoy :)