DigiCert are indeed our parter CA - they do Facebook, GitHub, Stripe and Yahoo's SSL certs. We looked at a number of CAs (and actually wrote code against their APIs when we were testing), and chose DigiCert based on a combination of EV verification time and business practices.
Our retail prices are the same as DigiCert's - I suspect you're looking at the 'per year' price on https://www.digicert.com/ev-ssl-certification.htm. With the HN discount, you get the ability to apply for a certificate in less than a minute combined with DigiCert's fast verification practices.
With CertSimple the big number is always the final price.
That includes unlimited server licenses, we don't try and upsell SGC, etc. We're actually really picky things like that, see: https://certsimple.com/about