I believe a warning message effects user experience, it is a unnecessary click and you should opt to use a ssl certification that does not create any warning messages.
You should always approach website design and functionality with the assumption that people know less than yourself, most people will not understand the message they are seeing and so many not even read the message and click off - creating a bounce before they even visit your site.
It's also the possibility that people who don't understand such warnings with associate the site with being harmful, Firefox for example gives a very alarming screen that is red and Id imagine a lot of people using Firefox would click away. For the sake of $10 a year its simply not even considering.