Can anyone direct me to a page that shows options and pricing for upgrading from Sonar Producer X3 to the latest versions? Can't seem to find it on the current site.

Earlier this year BandLab purchased Cakewalk IP from Gibson. They created Cakewalk By BandLab based on the last release of SONAR Platinum. CbB is free.
 
Leave X3 Producer installed to take advantage of the plug-ins bundled with it.
 
CbB installation instructions are here. It requires BandLab Assistant which further down the installation page. It also requires setting up an account at BandLab. Use the same email as your old Cakewalk Account so they can migrate your account information.
 
The CbB installation is similar to a SONAR upgrade, after running the CbB installer, both X3 and CbB will be available.