Backends clarification

[atarno]

hi folks,

i am afraid i misunderstand the concept of backends' pricing and need your help with this.
as far as i understood, backends' pricing is done per hour per instance.
for test purposes i created a b4 backend in my app. two instances have been loaded. after ~10 minutes i was out of promised free "9.00 Instance Hours" - red dashboard and no soup for me. what am i missing?

thanks.

[pdknsk]

The free allocation is $0.72 (or 9h of one B1 instance) per day. B4 is
expensive.

http://code.google.com/appengine/docs/billing.html#Billable_Resource_Unit_Cost