The $20 plan gives you ~120 minutes of high thinking time from Argon weekly. Thinking time isn't an exact metric, context size matters a lot, but I have been using it on the $20 and still have 25% left with ~85 minutes of thinking time on high for a project I have been working on.

I have Gemini because of my Google One membership. I already was on their 2TB plan and it gave me Pro level, IIRC, and it was generous. Now I'm on Ultra mostly because of Google Drive storage I get, but Gemini also gets a nice boost, as well.

I'm on $20 Claude plan, and it's also ample for my needs, for now.

I think this is because of capacity issues. Higher paying per token users probably get preferential capacity allocations to those on monthly plans. The allowance for what you get in the monthly plan increases as the models age and the likely capacity problems improve.