First off, you are right, don't write a post for free!
Second, like someone else said, you have to look at what you bring to the table, how far is your reach? How many views with the post get? Will you promote it on your social media? How new are you?
All of these things will help determine how much you can charge. I have 2 options, a product review in which the company sends me the product and I charge a nominal fee, and a sponsored post which they do not send the product to review, but costs more.
You may have to come down on your price a little for your first couple of posts just to show what you can do. Once you have a couple of examples, you can charge more.
The thing is products don't pay the bills, so you can charge to review a product or post a sponsored post.
Just know that there are companies out there who will try to get a link on your site for free, and often times that means making you feel bad for charging, but stick to your guns. Like I said, no matter how much the product costs, it won't pay the bills!