Earlier Scenario:
Dependent Postal (along with Main applicant) = 500 GBP
Dependent Postal (separately) = 550 GBP
This means it was cheaper to send along with Main applicant than separately.
Post 6th April:
Dependent Postal (along with Main applicant) = 750 GBP (+ 250 GBP)
Dependent Postal (separately) = 561 GBP (+ 11 GBP)
This means it is cheaper to send the dependent form separately now.
Who will do it at the same time.
No one will ever send the file together even if they are sending it the same day.
This is based upon the Payment Details page of the new Dependent form.
I think UKBA has missed the trick and gave applicants a reason to do it separately always.