I dunno, I think Inno's actually rather inconsistent with their diamond prices. Things like scouting, refreshing the Dealer (the first time), the collect all function, the 'premium' impediment removals in Egypt, and extra negotiation turns are all practically free. Even expansions for the most part are pretty cheap, and they're almost objectively the best thing to spend diamonds on (talking about the main city here of course, not settlements
).
But even with emissary slots relatively being a lot more expensive, I feel that for a one-time cost that extra emissary bonus will pay itself off eventually. It has to, right?