You can still use a free instance that is 6.5 hours a day, 1 gig of datastore, 1 gig each of incoming and outgoing bandwidth, etc. Low volume sites should easily be able to stay in the free zone.
Free instances had better have a low loading request time (e.g., for Java apps, use Objectify instead of JPA, etc.).
Assuming I understand the new billing correctly, I can get a paid plan for $9/month + $36/month for one reserved always on instance. I also get 24 hours/day of free on demand front end instances to handle a bit more traffic.
I think that a paid for plan still gets the other free quotas of a free plan and you pay each day for any use over each free limit. Is this right? It looks like you can have 1 always on FE instance, and 1 or 2 on demand FE instances, and a reasonable about of resource use for less than a $100/month.