How do I stop this from happing? Do I need to buy an SSL cert? Aren't these really expensive per server and I have 5 servers I would need to do this too.
Yes. You need one SSL certificate per site. Although I've never bought an SSL certificate, I have looked at www.rapidsslonline.com .. their rapid ssl is I think $18/year and less if you buy for multiple years. If all of your websites are under the same domain, you could buy a wildcard cert as well.
HTH, Barry