I have deduced that, mathematically, the first 24-person mount will cost $54.00 USD.

A one seater, account-wide mount is $24.

A two-seater, account-wide mount is $29.99. Effectively $5.99 for another person.

A four-seater, account-wide mount is $35.99. Again $6, but twice much of a gain in capacity when compared to the previous model for the same cost. Effectively $3/person.

And now the Lunar Whale, our first eight-seater, account-wide mount is $42.00 even. Once more, $6 up (well, $6.01), but this time for four seats, so $1.50/person.

[slaps whale] this bad boy can fit so much raid

Ergo since the next size up is a full Alliance, at 24 people, the next mount size increase follows logically at 16 more seats at $0.75 (since the cost per seat halves every time) per seat.