I use to run an ad server where numbers of requests like this per month would be considered low(we would regularly do hundreds of millions of requests per day peaking at around a billion per day). We had the added fun of requiring extremely low latency and we couldn't toss 400 servers at it either.

With chasing writes you do it in slow periods between traffic bursts since you're basically just pulling them off a queue to process so you don't need to count that in with your peak burst numbers.

Your costs seem really high too. The above system was ~$10K/month on GoGrid including 2 DB servers that were on dedicated servers(not really impressive ones either I think they were ~$500/month each), a load balancer on a dedicated server, 2 dozen webservers or so, a few support servers(admin panels, client interfaces, puppet, etc), and a small hadoop cluster.

Redis would receive the raw data, the DB stored the rolled up data and the raw data/logs would be compressed and go onto a small hadoop cluster in case we needed to process it for a new type or report or look for something specific.