But the ridiculous egress costs of the big clouds really reduce the feasibility of this. If you have some 'bare metal' boxes in the same city as your cloud instances you are going to be absolutely clobbered with the cost of database traffic from your additional AWS/azure/whatever boxes.
Is database traffic really all that significant in this scenario? I'd expect the bulk of the cost to be the end-user traffic (serving web pages to clients) with database/other traffic to your existing infra a relatively minor line-item?
