Let's say I am happy with the Linode 2GB plan except for the CPU Cores count.
I know that I can pay for additional: RAM, Disk Space, Transfer, and IP.

WHY THERE IS NO SUCH OPTION FOR CPU CORES ?

I believe that hardware-based extras are no longer available for hourly-billing linodes (which are the only option for new accounts going forward). So no extra RAM or disk space upgrades. Like CPU cores, you'll need to resize to a bigger linode for hardware upgrades.

This makes a lot of sense, as it simplifies management and allocations for Linode.

How about $30 plan which is the same as $20 plan but with 4 CPU cores

If you're lucky enough to be grandfathered in, the $30 plan actually only gets 3 cores.