Good low-cost SSL certificate providers [closed]
Solution 1:
The cheapest ones are not practically less secure than the expensive ones. Since they are for your own users, the extra marketing boost from a well-known name is wasted. The cheapest option from godaddy.com should work for your purposes.
Solution 2:
Check out http://www.startssl.com/ for some low price (and simple free options). I use the free version for my sites SSL. One caveat, is the certificates are already understood by Firefox, but IE warns that the certificate may come from an untrusted site.