In the distant past, there was an option to add RAM in 360 MB increments for $20/mo, if physically available. This option was removed some time ago. I do not recall it ever being less expensive than upgrading to the next higher plan.
The problem is that Linode can't easily sell "only" RAM or "only" storage without resources going to waste. There's enough RAM and enough storage for some number of Linodes on a server, and increasing either RAM or storage decreases the number of Linodes that can live on that host.
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