I had to work through the signed ClickOnce recently. It was not the most smooth experience, but once this has been done (and wired to the automated publishing) it started working like a marvel. Here’s the actual product, if you are interested in the behavior: Forecasting Add-in for Excel.
Article on ClickOnce Manifest Signing and Strong-Name Assembly Signing was a good start.
Can I use the same certificate to sign multiple ClickOnce assemblies?
Yes.
How much does a certificate cost?
Tucows offers certificates for 75$, but the process is rather slow.
What information do I need to provide when obtaining the certificate?
They check your documents (e.g. a scan of your certificate of incorporation), check your whois record and things like this.
Does the certificate need to be aware of where the publishing location is?
This was not needed in our case.