You can put a paypal donate button here, but you cannot as far as I know do any sort of paid subscription since the only way to do that would be to make your site private, or password protect all posts and pages. The private blog is no good since no one would be able to get to the blog to even pay, and the password protection thing is no good since one person could pay and then give the password to 50 friends, or post it on facebook or twitter and all of a sudden you have 5,000 visitors and only 10 paid.
If you want a paid subscription site, you will have to go with a self-hosted wordpress installation at a third-party hosting service such as godaddy, bluehost, etc. Figure that will cost an average total of about $10 to $25 per month. There are some membership plugins for self-hosted sites, but most require you to pay a fee if you want some of the advanced features and can cost anywhere from $99 to about $450 per year.
Here is the support document on the paypal donate button.
If you are going to attempt to do some sort of paid subscription thing you really need to run that past staff since as I said, it might be against the Terms of Service here.