I personally use App Engine as It allows (or rather, allowed) me to concentrate on product (in theory), rather than itsy bitsy stuff
Up to now I ignored the Stackdriver emails, and only now checked what actually it was telling (Ignored because I'm here for the PaaS, If I was going to use multiple services individually, I'd have invested my efforts in AWS back in the day)
Anyway, long story short, my 3 main projects seem to be digesting 230, 460, and 1300GB's monthly, as I understand, I'll start paying $0.5 per GB now, which makes $995 monthly, considering I paid ~$600/mo for everything up to now, bumping that up to $1600 seems extreme, I might be doing the calculation wrong, or I might be logging too much stuff
Anyway, my thoughts are negative (Appreciate the very very very early warnings, and the effort, yet inspecting this last email, investing 15 minutes into understanding things, It's not enough to grasp the change in pricing, as an example, the link in the email that's supposed to bring you to "Billable log volume across a billing account available" - directs you to main billing page - Just a summarised, hey, here's what you paid, and here's what you'll pay now email would be the best)