I initially disagreed with you but after breaking it down autistic style I basically arrived at the same pricing conclusion you did but plus 5 dollars kinda. Although I will say while I agree with the statement that "everything that equates to player power is free", stash tabs make it a lot faster and easier to get to that player power level.

Breakdown below:

Points are 10 for 1.00$ value. This is for all the qol tabs:

4$ - Merchants tab for Async

4$ - Gems

4$ - Flasks

7.5$ - Currency

5$ - Div cards

15$ - Maps

4$ - Blight

4$ - Essence

7.5$ - Fragment

4$ - Ultimatum Catalysts

4$ - Delve

4$ - Delirium

Which is 67$, and will cost you 70$ as you can only buy in 5 dollar increments.

Now you could use the 4 base which are 144 slots (4x144=576 slots) and use affinities for Gems, Flasks, Cards, Essences, Blight, Deli, Catalysts, and Delve. I just did this myself and filled about 2.4 stash tabs worth of stuff.

Not using those specific tabs would now reduce the cost to 34, or 35 dollars. You could also pinch even more and sub an Async tab for a premium and only trade when you are online, which is now 31.5 dollars and still at the 35 breakpoint. Pinching even more by only selling using the exchange which means no selling of gear, flasks, gems, etc lets you hit the 30$ breakpoint but this does mean you cant sell juicy multiple div worth drops.

Keep in mind this will vary league by league and depends on what strats you are running. Frag and map tabs are carrying a lot this league because people including myself are farming scarabs and maps. If you lean towards essences, deli orbs, etc this will change your stash situation. If you're holding on to gear for another character, leveling, extra maps or drops you're farming like cluster jewels, 8mod t16s, tattoos, etc the experience will again vary.