I use a Global Sign ID (your last link). I'm not aware of anything significantly less expensive, though it's been a while since I've checked. For what it provides (hardware-based token, secure timestamp server, user identity verification, etc.), the cost seems reasonable as a business expense. There is real work and expenses involved for a signing authority to set up and maintain the infrastructure that makes it all work.
Self-signed certificates can work if you can get those who wish to validate your digital signatures to add your certificate to their list of trusted certificates, though you won't have certain things like secure timestamps.
Another option that might work for you is Adobe Sign, which is included with Acrobat DC. You could sign your own documents and the resulting signed PDF is digitally signed by the service.
I follow you on the cost. In terms of how frequently I'd use it, you're correct - it's not too steep.
I actually tried sending through Adobe Sign and cc'd myself and the signature validation status came through as "Signature Validity Unknown".
I'm not sure if you mean that the document itself is signed somewhere other than the signature box I had inserted in the .pdf prior to signing and sending?
Thanks again for your reply.
Yes, I meant the entire document is digitally signed by Adobe.