Effects and their costs can't really be separated from each other or from game context. The mana curve very much dictates the direction of Vintage, even if it isn't explicit. Nowhere is this more obvious than the decline of Mana Drain: such a powerful abstract effect, but the classic Mox-powered combo control strategy that wants it can't use it favorably against the early threats, cheap interaction or taxing effects that sprung up around it in the last 3 or so years.
We sometimes temporarily set this reality aside to evaluate the most unique effects because they don't have many comparables. I'm thinking of certain one- or two-card combos in smaller formats, or Time Walk effects in Vintage. For the most part though, comparables are really useful.
I remember thinking about a year after JTMS took over Vintage that the right way to have looked at it at the time was a weaker but easier to facilitate Necropotence, which the combo-control strategies at the time salivated over. Nowadays this analogy breaks down in many ways, mostly because less blowout effects like Tinker, Key/Vault, etc. are being played. As a combination quality/removal engine, the Mind Sculptor tends to come down too late in a lot of matchups compared to e.g. JVP.
Back to Ajani: as above, the gold standard for quality/removal is JVP. It is really hard to justify going up the curve much further if what you want to achieve is multiplexing removal onto your value engine, since a Swords alongside JVP costs just 1 mana on both the front and back sides. The premium I currently pay to get more value is 1 mana to upgrade to Dack (in Gush aggro shells against control opponents) or maybe 2 mana to JTMS (as the upper end of Oath combo-control). But I wouldn't play Ajani over either of the above unless I really valued having direct (instead of incidental) access to the Swords effect. And if that was the case I wouldn't pay more than two mana, because in this metagame if you can't access your Swords effect on the second turn you might as well not have a Swords effect. The only exception were if I playing a control deck that really valued a fifth and sixth copy of Swords for late game TKS/Smasher. Then I might swallow playing 3 mana; but in such a deck would I even get much out of the +2?
In sum: no more than two mana in the current metagame.