There is no one definitive answer to this question. Every situation is different and will require a different price tag. However, a good starting point would be to ask what the price would be for an article of similar length, quality and scope. Additionally, you should also consider the time commitment that the writer will need to put in.
If the blog post is lengthy or complex, the price may be higher. Conversely, if the post is shorter or less involved, the price may be lower. Ultimately, it is important to get an estimate before agreeing to pay anything, in order to ensure that you are getting what you expect and that the writer is worth their fee.