So far:

single-character mounts: $12
account-wide mounts: $24
Speedy fan fest account-wide mount and two-seater account-wide mounts: $29.99
four-seater account-wide mount: $35.99

So, it has gone roughly $6 from 1-seater to 2-seater and another $6 from 2-seater to 4-seater.

If the pattern holds, my guess would be:

eight-seater fan fest (account-wide?) mount: $41.99